Induced Currents
Chapter XIV. Induced Currents.
At the present time, voltaic cells produce but a very small part of the current electricity used. Practically all that is employed for power, light, heat, and electrolysis is produced by the use of magnetic fields, or by electromagnetic induction.
297. Laws of Induced Currents.[M]?To illustrate how a current can be produced by electromagnetic induction:
Fig. 279.?The moving magnet induces a current in the coil.
Fig. 280.?A current may be induced by turning the coil in the earth's magnetic field.
298. The magneto is a device that illustrates the laws of induced currents stated in Art. 297. The magneto (see Fig. 281), consists of several permanent, "U"-shaped magnets placed side by side. Between the poles of these magnets is placed a slotted iron cylinder having a coil of many turns of fine insulated copper wire wound in the slot as in Fig. 282. The cylinder and coil form what is called an armature. The armature is mounted so as to be revolved between the poles of the "U"-shaped magnets by means of a handle. As the armature revolves, the lines of force from the magnets pass through the coil first in one direction and then in the other. This repeated change in the lines of force passing through the coil produces an E.M.F. which may be felt by holding in the hands the two wires leading from the armature coil. On turning the armature faster the current is felt much stronger, showing that the E.M.F. in the coil increases as the rate of cutting the magnetic lines of force by the coils increases.
Fig. 281.?A magneto.
Fig. 282.?A shuttle armature.
Fig. 283.?The induced current has a field which opposes the motion of the magnet. The heavy line represents the direction of the induced current.
299. Lenz's Law.?While one is turning the armature of a magneto if the two wires leading from its coil are connected, forming what is called a "short circuit," the difficulty of turning the armature is at once increased. If now the circuit is broken, the armature turns as easily[Pg 329] as at first. The increased difficulty in turning the armature is due to the current produced in the coil. This current sets up a magnetic field of its own that opposes the field from the steel magnets. This opposition makes it necessary for work to be done to keep up the motion of the coil when a current is passing through it. This fact is called Lenz's Law. It may be expressed as follows: Whenever a current is induced by the relative motion of a magnetic field and a conductor, the direction of the induced current is always such as to set up a magnetic field that opposes the motion. Lenz's Law follows from the principle of conservation of energy, that energy can be produced only from an expenditure of other energy. Now since an electric current possesses energy, such a current can be produced only by doing mechanical work or by expending some other form of energy. To illustrate Lenz's Law, suppose that the north-seeking pole of a bar magnet be inserted in a closed coil of wire. (See Fig. 283.) The current induced in the coil has a direction such that its lines[Pg 330] of force will pass within the coil so as to oppose the field of the bar magnet, when the north pole of the magnet is inserted so as to point to the left. That is, the north pole of the helix is at the right. Applying the right-hand rule to the coil, its current will then be counter clockwise. On withdrawing the magnet, the current reverses, becoming clockwise with its field passing to the left within the coil.
A striking illustration of the opposition offered by the field of the induced current to that of the inducing field is afforded by taking a strong electromagnet (see Fig. 284) and suspending a sheet of copper so as to swing freely between the poles. When no current flows through the magnet the sheet swings easily for some time. When, however, the coils are magnetized, the copper sheet has induced within it, currents that set up magnetic fields strongly opposing the motion, the swinging being stopped almost instantly. The principle is applied in good ammeters and voltmeters to prevent the swinging of the needle when deflected. The current induced in the metal form on which is wound the galvanometer coil is sufficient to make the needle practically "dead beat."
Fig. 284.?The magnetic field stops the swinging of the sheet of copper.
300. The Magneto and the Dynamo.?Magnetos are used to develop small currents, such as are used for telephone signals, and for operating the sparking devices of gasoline engines. They are therefore found in automobiles containing gasoline motors. The most important device for producing electric currents by electromagnetic induction, however, is the dynamo. It is employed whenever large currents are desired. The principle of this device is similar to that of the magneto except that it contains[Pg 331]
[Pg 332]
[Pg 333] an electromagnet for producing the magnetic field. Since the electromagnet can develop a much stronger field than a permanent magnet, the dynamo can produce a higher E.M.F. and a much larger current than the magneto.
Lord Kelvin
"By Permission of the Berlin Photographic Co., New York."
Lord Kelvin (Sir William Thomson), (1824-1907). Professor of Physics, Glasgow University. Invented the absolute scale of temperature: also many practical electrical measuring instruments. The foremost physicist of the latter part of the nineteenth century.
Michael Faraday
"By Permission of the Berlin Photographic Co., New York."
Michael Faraday (1791-1867). Famous English Physicist. Made many discoveries in electricity and magnetism; "Greatest experimentalist of the nineteenth century."
301. The Magnetic Fields of Generators.?In the magneto, the magnetic field is produced by permanent steel magnets. In dynamos powerful electromagnets are used. The latter are sometimes excited by currents from some other source, but usually current from the armature is sent around the field coils to produce the magnetic fields. Dynamos are classified according to the manner in which the current is sent to their field coils.
Fig. 285.?A series-wound dynamo.
Fig. 286.?A shunt-wound dynamo.
Fig. 287.?A compound-wound dynamo.
A. The series wound dynamo (see Fig. 285) is arranged so that all of the current produced by the armature is sent through coils of coarse wire upon the fields, after flowing through the external circuit.
B. The shunt wound dynamo (see Fig. 286) sends a part only of the current produced through the field coils. The latter are of many turns of fine wire so as to use as little current as possible. The greater part of the current goes to the main circuit. If the number of lamps or motors[Pg 334] connected to the main circuit is increased, the voltage is lessened which weakens the current in the field coils, causing a weaker field and still lower voltage, producing a fluctuating E.M.F. which is unsatisfactory for many purposes. This fault is overcome by
C. the compound wound dynamo. This dynamo has both shunt and series coils upon its fields. (See Fig. 287.) If more current is drawn into the main circuit with this dynamo, the series coils produce a stronger field compensating for the weaker field of the shunt coils, so that uniform voltage is maintained. The compound wound generator is therefore the one most commonly employed.
2. Devices, (a) magneto, (b) dynamo: series, shunt, compound.
3. Illustrations of the laws.
2. Show how Lenz's Law, follows from the principle of conservation of energy.
3. A bar magnet is fixed upright with its north-seeking pole upward. A coil is thrust down over the magnet. What is the direction of the current induced in the coil? Explain.
4. In what two ways may a current be induced in a closed coil?
5. What method is employed in the magneto? In the dynamo?
6. What is the nature of the current produced in the armature coil of a magneto, that is, is it direct or alternating? Why?
7. What is the resistance of a 20-watt tungsten lamp if the E.M.F. is 115 volts?
8. Find the resistance of a 40-watt tungsten lamp when the voltage is 115? How much heat will it produce per minute?
9. An Edison storage battery cell on a test gave a discharge of 30 amperes. The average voltage was 1.19. What was the resistance of the cell?
[Pg 335]
10. Eight storage cells are connected in series. Each has an E.M.F. of 1.2 volts and an internal resistance of 0.03 ohms. What will be the current flowing through a voltmeter having 500 ohms resistance in circuit with them?
Fig. 288.?Several methods of arranging the field coils and the armature of a dynamo.
Fig. 289.?A drum armature.
The field coils vary in number and position. The purpose of their construction is always to send the largest possible number of lines of force through the armature. Some dynamos are bipolar, or have two poles, others are multipolar or have more than two. In Fig. 288 No. 4 has four poles. The armature of a dynamo differs from a magneto armature in that it consists of a series of coils of[Pg 336] insulated copper wire wound in numerous slots cut in the surface of a cylindrical piece of iron. Fig. 289 shows a side view of the iron core of such an armature. Iron is used to form the body of the armature since the magnetic lines of force flow easily through the iron. The iron by its permeability also concentrates and increases the magnetic flux. The best armatures are made of many thin sheets of soft iron. These are called laminated armatures. An armature made of a solid piece of iron becomes hot when revolving in a magnetic field. This is due to electric currents induced in the iron itself. This heating is largely reduced by laminating the armature. Why?
Fig. 290.?Armature connected to slip rings producing an alternating current.
303. Methods of Collecting Current from the Armature.?The electric currents produced in the armature are conducted away by special sliding contacts. The stationary part of the sliding contact is called a brush. The moving part is a slip ring or a commutator. Fig. 290 shows an armature coil connected to slip rings. As the armature revolves, the coils and slip rings revolve with it. The two ends of the armature coils are connected to the two rings respectively. Now as the armature revolves it cuts the lines of force first in one direction and then in the other. This produces in the coils an E.M.F. first one way and then the other. This E.M.F. sets up a current which is conducted to the outside circuits through the slip rings and[Pg 337] brushes. Such a current which repeatedly reverses its direction is called an alternating current. Fig. 291 (1) indicates graphically how the current moves alternately one way and then the other. Alternating currents are extensively used for electric light, heat, and power. Direct currents or those going continuously in one direction are however in much demand especially for street car service, for electrolysis, and for charging storage batteries.
304. The Commutator.?For a dynamo to deliver a direct current it must carry upon the shaft of the armature a commutator. The commutator is used to reverse the connections of the ends of the armature coils at the instant that the current changes its direction in the armature. This reversal of connection when the direction of current changes, keeps the current in the outside circuit flowing in the same direction. Fig. 291 is a diagram of an armature with a commutator. The commutator is a split ring, having as many parts or segments as there are coils upon the armature. The brushes touch opposite points upon the commutator as they slide over the surface of the latter. Suppose that the armature viewed from the commutator end rotates in a counter-clockwise direction, also that the currents from the upper part move toward the commutator and out the top brush.
Fig. 291.?The armature coils are connected to a commutator producing a direct current.
As the armature revolves, its coils soon begin to cut the force lines in the opposite direction. This change in the direction of cutting the lines of force causes the current to reverse in the coils of the armature. At the instant the current changes in direction, what was the upper segment[Pg 338] of the commutator slips over into contact with the lower brush, and the other segment swings over to touch the upper brush. Since the current has reversed in the coils it continues to flow out of the upper brush. This change in connection at the brushes takes place at each half turn of the armature, just as the current changes in direction in the coils. This is the manner in which the commutator of a dynamo changes the alternating current produced in the armature coils, into a direct current in the external circuit. Fig. 292 (1) represents graphically an alternating current, (2) of the same figure shows current taken from the brushes of the commutator of a dynamo with one coil on the armature.
Fig. 292.?Graphic representation of (1) an alternating current; (2) a pulsating current; (3) a continuous current.
Fig. 293.?DeLaval multi-stage turbine and gear driving 750-kw., 750-r.p.m., 600-volt direct-current generator.
A practical dynamo, however, has many coils upon its[Pg 339] armature with a corresponding number of segments upon the commutator. (See Figs. 289 and 293.) As each coil and commutator segment passes a brush, it contributes an impulse to the current with the result that armatures with many coils produce currents that flow quite evenly. (See Fig. 292, 3.)
The current represented in Fig. 292 (2) is called a pulsating current.
Fig. 294.?A wire carrying a current across a magnetic field is pushed sideways by the field.
305. The electric motor is a machine which transforms the energy of an electric current into mechanical energy or motion. The direct current motor consists of the same essential parts as a direct current dynamo, viz., the field magnet, armature, commutator and brushes. Its operation is readily comprehended after one understands the following experiment:
Set up two bar electromagnets with unlike poles facing each other about an inch apart. A wire connected to a source of current is hung loosely between the poles as in Fig. 294. The circuit through the wire should contain a key or switch. If a current is sent through the electromagnets and then another is sent through the wire, the[Pg 340] latter will be found to be pushed either up or down, while if the current is reversed through the wire it is pushed in the opposite direction. These results may be explained as follows:
Consider the magnetic field about a wire carrying a current (See Fig. 295.) If such a wire is placed in the magnetic field between two opposite poles of an electromagnet (Fig. 296), the wire will be moved either up or down. The reason for this is shown by the diagram in Fig. 297. Here a wire carrying a current and therefore surrounded by a magnetic field passes across another magnetic field. The two fields affect each other causing a crowding of the force lines either above or below the wire. The wire at once tends to move sideways across the field away from the crowded side. In the figure, the wire tends to move downward.
Fig. 295.?The magnetic field about a wire carrying a current.
Fig. 296.?The magnetic field between two unlike poles.
Fig. 297.?The crowding of the lines of force above the wire, pushes it downward.
In a practical motor, the wires upon the armature are so connected that those upon one side (see Fig. 298), carry currents that pass in, while on the other side they pass out. To represent the direction of the current in the wires, the[Pg 341] following device is employed; a circle with a cross (to represent the feather in the tail of an arrow) indicates a current going away from the observer, while a circle with a dot at its center (to represent the tip of an arrow) indicates a current coming toward the observer.
Fig. 298.?The crowding of the lines of force causes the armature to revolve in a clockwise direction.
In Fig. 298 the north pole is at the left and the south pole at the right. The field of the magnets therefore passes from left to right as indicated in the figure. Now in the armature the currents in the wires on the left half of the armature are coming toward the observer while those on the right move away. Applying the right-hand rule, the magnetic lines will crowd under the wires on the left side of the armature while they will crowd over the wires on the right side. This will cause a rotation up on the left side and down on the right, or in a clockwise direction.
Fig. 299.?View of a one-half horse-power motor.
If the current in the armature is reversed (in on the left and out on the right), the lines of force will crowd the armature around in the opposite direction or counter clockwise. The rotation of the[Pg 342] armature will also be reversed if, while the current in the armature is unchanged in direction, the poles of the magnet are changed thus reversing the magnetic field.
The motorman of a street car reverses the motion of his car by reversing the direction of the current in the armature of the motor.
Fig. 300.?The frame and electromagnet (at left), front bracket and brush holder (at right) of the motor shown in Fig. 299.
Fig. 301.?The armature of a motor.
306. Practical motors have many coils upon the armature with a corresponding number of segments upon the commutator. A large number of coils and commutator segments enables some one of the coils to exert its greatest efficiency at each instant, hence a steady force is provided for turning the armature which causes it to run smoothly.[Pg 343] Fig. 299 represents a 1/2 horse-power motor ready for use while Fig. 300 shows the frame and poles and the front bracket and brush holder, and Fig. 301 represents the armature.
2. The electric motor: (a) essential parts, (b) action.
2. How is this current produced? Give careful explanations.
3. What is the result of Lenz's law as applied to the dynamo?
4. Apply the first two laws of electromagnetic induction to the dynamo.
5. What is the power of a dynamo if it produces 40 amperes of current at 110 volts?
6. How much power must be applied to this dynamo if its efficiency is 90 per cent.?
7. A motor takes 10 amperes of current at 220 volts; what is the power of the current in watts? If this motor has an efficiency of 95 per cent., how many horse-power of mechanical energy can it develop?
8. Explain why reversing the current in the armature of a motor reverses the direction of rotation.
9. Find the cost of running a washing machine using a 1/2-horsepower motor 2 hours if the cost of the electricity is 10 cents a kilowatt hour.
10. A 1/8-horse-power motor is used to run a sewing machine. If used for 3 hours what will be the cost at 11 cents a kilowatt hour?
Fig. 302.?An induction coil.
The current from the battery flows through the primary coil magnetizing the iron core. The magnetism in the core attracts the soft-iron end of the interrupter, drawing the latter over and breaking the circuit at the screw contact, K. This abruptly stops the current and at once the core loses its magnetism. The spring support of the interrupter now draws the latter back to the contact, T, again completing the circuit. The whole operation is repeated, the interrupter vibrating rapidly continually opening and closing the circuit.
Fig. 303.?Diagram showing the parts of an induction coil.
308. The Production of Induced Currents in the Secondary Coil.?When the current flows through the[Pg 345] primary it sets up a magnetic field in the core. When the current is interrupted, the field disappears. The increase and decrease in the field of the core induces an E.M.F. in the secondary coil, in accordance with the first law of electromagnetic induction. The E.M.F. produced depends upon (a) the number of turns in the secondary, (b) the strength of the magnetic field and (c) the rate of change of the field. The rate of change in the field is more rapid at the break than at the make. When the circuit is closed it takes perhaps 1/10 of a second for the current to build up to its full strength while at a break the current stops in perhaps 0.00001 of a second, so that the induced E.M.F. is perhaps 10,000 times as great at "break" as at make. To increase the suddenness of the "make" and "break," a condenser is often connected in the primary circuit, in parallel, with the interrupter. (See Fig. 303, C.) This condenser provides a place to hold the rush of current at the instant that the interrupter breaks the circuit. This stored up charge reinforces the current at the make producing a much more sudden change in the magnetic field with a corresponding increase in the E.M.F. The induced currents from induction coils are sometimes called faradic currents in honor of Faraday who discovered electromagnetic induction. They are used to operate sparking devices upon gas and gasoline engines and in many devices and experiments in which high-tension electricity is employed.
Fig. 304.?The transformer has a closed core; the induction coil, an open core.
Fig. 305.?The laminated iron core of a transformer.
Fig. 306.?Cross-section of the transformer shown in Fig. 305 showing the magnetic field around the primary and secondary coils.
309. The Transformer.?This is like the induction coil in that it uses a primary and a secondary coil, and an iron core to carry the magnetic field. (See Fig. 304.) They differ in that the transformer has a closed core or one forming a continuous iron circuit, while the induction coil has an open core, or one in which the magnetic field must travel in air from the north to the south poles of the core. The[Pg 346] transformer must always be used with an alternating current while the induction coil may use either a direct or an alternating current. Further, the induction coil always produces a higher E.M.F. while the transformer may produce an E.M.F. in its secondary coil that is either higher or lower than the one in the primary. The former is called "step-up" while the latter is a "step-down" transformer. The alternating current in the primary coil of the transformer produces an alternating magnetic flux in the iron core. This iron core is laminated (see Fig. 305) to prevent the heating that would result if a solid core were used. The alternating magnetic flux induces in the secondary coil an E.M.F. in accordance with the following rule. The[Pg 347] ratio of the number of turns in the primary to the number of the turns in the secondary coil equals the ratio of the electromotive forces in these respective coils. If the secondary coil has 8 turns while the primary has 4, the E.M.F. of the secondary will be just twice that of the primary. Or, if in the primary coil of the transformer Fig. 306 is an E.M.F. of 110 volts, in the secondary will be found an E.M.F. of 220 volts.
Fig. 307.?A commercial transformer.
310. Uses of Transformers.?In electric lighting systems, dynamos often produce alternating currents at 1000 to 12,000 volts pressure. It is very dangerous to admit currents at this pressure into dwellings and business houses, so that transformers are installed just outside of buildings to "step-down" the high voltage currents to 110 or 220 volts. The lighting current that enters a house does not come directly from a dynamo. It is an induced current produced by a transformer placed near the house. (See Fig. 307.) In a perfect transformer the efficiency would be 100 per cent. This signifies that the energy that is sent into the primary coil of the transformer exactly equals the energy in the secondary coil. The best transformers actually show efficiencies better than 97 per cent. The lost energy appears as heat in the transformer. "The transfer of great power in a large transformer from one circuit to another circuit entirely separate and distinct, without any motion or noise and almost without loss, is one of the most wonderful phenomena under the control of man."
311. The mercury arc rectifier is a device for changing an alternating current into a direct current. It is frequently used for charging storage batteries where only alternating current is supplied by the electric power company.[Pg 348] It consists of an exhausted bulb containing two carbon or graphite electrodes marked G in Fig. 308 and a mercury electrode marked M. It is found that current will pass through such a bulb only from the graphite to the mercury but not in the reverse direction. In operating the device, the secondary terminals of an alternating current transformer T are connected to the graphite terminals of the rectifier. A wire connected to the center of the secondary of the transformer at C is attached to the negative terminal of the storage battery SB. The positive terminal of the battery is connected to the mercury electrode of the rectifier tube through a reactance or choke coil R. This coil serves to sustain the arc between the alternations. Sw is a starting switch, used only in striking the arc. It is opened immediately after the tube begins to glow.
Fig. 308.?Diagram of a mercury arc rectifier.
2. What must be the relative number of turns upon the primary and secondary coils of a transformer if it receives current at[Pg 349] 220 volts and delivers current at 110? Also show by diagram.
3. Would the transformer work upon a direct current? Why?
4. Explain why the interrupter is a necessary part of the induction coil and not of the transformer.
5. If a building used eighty 110-volt incandescent lamps, what would be necessary to light them if they were joined in series? Why would this not be practical?
6. If a 16-candle-power lamp requires 0.5 ampere upon a 110-volt circuit what current and voltage will be needed to operate 12 such lamps in parallel?
7. What will it cost to run these lamps 4 hours a night for 30 days at 10 cents per kilowatt hour?
8. If a mercury arc rectifier uses 5 amperes of current at 110 volts alternating current to produce 5 amperes of direct current at 70 volts, what is the efficiency of the rectifier?
9. Compute the heat produced in a 40 watt tungsten lamp in 1 minute.
10. Compute the heat produced in a 60 watt carbon incandescent lamp in 1 hour.
313. The telephone receiver was invented in 1876 by Alexander Graham Bell. It consists of a permanent steel magnet, U shaped, with a coil of fine insulated copper wire about each pole. (See Fig. 310.) A disc of thin sheet iron is supported so that its center does not quite touch the[Pg 350] poles of the magnet. A hard rubber cap or ear piece with an opening at its center is screwed on so as to hold the iron disc firmly in place.
Fig. 309.?The simplest telephone system. It consists of two telephone receivers connected in series on a circuit. It will work, but not satisfactorily.
The action of the receiver may be understood from the following explanation: The electric current sent to the receiver, comes from the secondary coil of the induction coil; it is an alternating current, fluctuating back and forth just in time with the waves of the voice affecting it at the transmitter. This alternating current flows around the coils on the poles of the permanent magnet. When this current flows in one direction, its magnetic field assists the field of the permanent magnet, strengthening it. This stronger magnetic field draws the thin iron disc in front of the poles of the magnet a little closer to them. When the current in the coils flows the other way, its magnetic field weakens the field of the steel magnet, and the disc is drawn back by the force of its own elasticity. Thus the disc of the receiver vibrates with the alternations of the current, and reproduces the same sounds that were spoken into the transmitter.
Fig. 310.?A telephone receiver. This receiver has a permanent horseshoe magnet with a coil about each pole.
314. The Telephone Transmitter.?The telephone receiver just described has great sensitiveness in reproducing sound, but it is not satisfactory as a transmitter or sending apparatus. The transmitter commonly used is represented in cross-section in Fig. 311. In this figure, back of the mouthpiece, is a thin carbon disc, D. Back of this[Pg 351] disc is a circular compartment containing granular carbon, g. The wires of the circuit are connected to the carbon disc and to the back of the case containing granular carbon. The circuit through the transmitter also includes a voltaic or storage cell and the primary coil of an induction coil. (See Fig. 312.)
Fig. 311.?A telephone transmitter.
Fig. 312.?Telephone instruments at one end of a talking circuit.
315. The action of the transmitter is explained as follows: When the sound waves of the voice strike upon the carbon disc, the latter vibrates, alternately increasing and decreasing the pressure upon the granular carbon. When the pressure increases, the electrical resistance of the granular carbon is lessened, and when the pressure upon it is decreased, its resistance increases. This changing resistance causes fluctuations in the electric current that correspond exactly with the sound waves of the voice affecting it.
316. A complete telephone system operating with a local battery is shown in Fig. 312. A person speaking into the transmitter causes a fluctuation in the electric current in the transmitter as described in Art. 315. This fluctuating current passes through the primary coil of the induction coil Ic. This fluctuating current produces a fluctuating magnetic field in its core. This fluctuating field induces an alternating current in the secondary coil which alternates just as the primary current fluctuates,[Pg 352] but with a much higher E.M.F. than the latter. The alternating current passes to the receiver which reproduces the speech as described in Art. 313. The line circuit includes the secondary of the induction coil, the receiving instrument and the receiver of the sending instrument so that the voice is reproduced in both receivers. An electric bell is placed at each station to call the attention of parties wanted. The movement of the receiver hook when the receiver is lifted, disconnects the bell and closes the talking circuit. The latter is opened and the bell connected when the receiver is hung up again.
Fig. 313.?Diagram of a telephone system as used in a large exchange.
In cities and towns, the telephone system in use differs from the one described in usually having one large battery placed in the central exchange, instead of dry cells at each instrument. (See Fig. 313.) Also the operator at central is called by simply taking the receiver from the hook instead of being "rung up" by the subscriber. The operations of the transmitter, induction coil and receiver, however, are the same in all telephones.
2. Transmitter: parts, action.
3. Induction coil, bell, line wires, etc.
4. Action of the whole device.
[Pg 353]
2. Connect the binding posts of a telephone receiver with a sensitive galvanometer and press on the diaphragm of the receiver; a deflection of the galvanometer will be noticed. Release the diaphragm and a reflection in the opposite direction is seen. Explain.
3. Is the current passing through the transmitter the one going to the receiver of the instrument? Explain.
4. Does the receiver at the telephone used by a person repeat the speech of the person? Explain.
5. How many 0.5 ampere lamps can be used with a 6 ampere fuse?
6. Why is it necessary to have a rheostat connected in series with a stereopticon or moving picture machine while a rheostat is not used with arc lights out doors?
7. How many candle power should a 60 watt carbon incandescent lamp give, if its efficiency is 3.4 watts per candle power?
8. Three incandescent lamps having resistances of 100, 150, and 240 ohms, respectively, are connected in parallel. What is their combined resistance?
Construction, action, and uses of?magneto, dynamo, induction coil, transformer, motor, telephone. Mercury arc rectifier.
Terms?primary, secondary, for coils and currents, armature, commutator, slip ring, brush, rectifier, open core, series, shunt, and compound connections for dynamos.
- (1) Electromagnetic Induction
- (2) The Dynamo and the Motor
- (3) The Induction Coil and the Transformer
- (4) The Telephone
(1) Electromagnetic Induction
296. Current Induced by a Magnet.?The discovery in 1819 that a current in a conductor can deflect a magnetic needle or that it has a magnetic effect, led to many attempts to produce an electric current by means of a magnet. It was not until about 1831, however, that Joseph Henry in America and Michael Faraday in England, independently discovered how to accomplish this important result.At the present time, voltaic cells produce but a very small part of the current electricity used. Practically all that is employed for power, light, heat, and electrolysis is produced by the use of magnetic fields, or by electromagnetic induction.
297. Laws of Induced Currents.[M]?To illustrate how a current can be produced by electromagnetic induction:
Connect a coil of 400 or more turns of No. 22 insulated copper wire to a sensitive galvanometer. (See Fig. 279.) Now insert a bar magnet in the coil. A sudden movement of the galvanometer will be noticed, indicating the production of a current. When the magnet stops moving, however, the current stops, and the coil of the galvanometer returns to its first position. If now the magnet is removed, a movement of the galvanometer coil in the opposite direction is noticed. This action may be repeated as often as desired with similar results.
Careful experiments have shown that it is the magnetic field of the magnet that produces the action, and that[Pg 327] only when the number of lines of force in the coil is changing do we find a current produced in the coil. These facts lead to Law I. Any change in the number of magnetic lines of force passing through or cut by a coil will produce an electromotive force in the coil. In the account of the experiment just given, electric currents are produced, while in Law I, electromotive forces are mentioned. This difference is due to the fact that an E.M.F. is always produced in a coil when the magnetic field within it is changed, while a current is found only when the coil is part of a closed circuit. The inductive action of the earth's magnetic field (see Fig. 280), may be shown by means of a coil of 400 to 500 turns a foot in diameter. Fig. 279.?The moving magnet induces a current in the coil.Fig. 280.?A current may be induced by turning the coil in the earth's magnetic field.
Connect its ends to a sensitive galvanometer and hold it at right angles to the earth's field. Then quickly revolve the coil through 180 degrees and note the movement of the galvanometer. Reverse the coil and the galvanometer swings in the opposite direction.
If the magnet in Fig. 279 is moved in and out of the coil at first slowly and later swiftly, small and large deflections of the galvanometer coil are noticed. The quicker the movement of the magnetic field the greater are the galvanometer deflections produced. This leads to Law II.[Pg 328] The electromotive forces produced are proportional to the number of lines of force cut per second.298. The magneto is a device that illustrates the laws of induced currents stated in Art. 297. The magneto (see Fig. 281), consists of several permanent, "U"-shaped magnets placed side by side. Between the poles of these magnets is placed a slotted iron cylinder having a coil of many turns of fine insulated copper wire wound in the slot as in Fig. 282. The cylinder and coil form what is called an armature. The armature is mounted so as to be revolved between the poles of the "U"-shaped magnets by means of a handle. As the armature revolves, the lines of force from the magnets pass through the coil first in one direction and then in the other. This repeated change in the lines of force passing through the coil produces an E.M.F. which may be felt by holding in the hands the two wires leading from the armature coil. On turning the armature faster the current is felt much stronger, showing that the E.M.F. in the coil increases as the rate of cutting the magnetic lines of force by the coils increases.
Fig. 281.?A magneto.Fig. 282.?A shuttle armature.
Fig. 283.?The induced current has a field which opposes the motion of the magnet. The heavy line represents the direction of the induced current. A striking illustration of the opposition offered by the field of the induced current to that of the inducing field is afforded by taking a strong electromagnet (see Fig. 284) and suspending a sheet of copper so as to swing freely between the poles. When no current flows through the magnet the sheet swings easily for some time. When, however, the coils are magnetized, the copper sheet has induced within it, currents that set up magnetic fields strongly opposing the motion, the swinging being stopped almost instantly. The principle is applied in good ammeters and voltmeters to prevent the swinging of the needle when deflected. The current induced in the metal form on which is wound the galvanometer coil is sufficient to make the needle practically "dead beat."
Fig. 284.?The magnetic field stops the swinging of the sheet of copper. [Pg 332]
[Pg 333] an electromagnet for producing the magnetic field. Since the electromagnet can develop a much stronger field than a permanent magnet, the dynamo can produce a higher E.M.F. and a much larger current than the magneto.
Lord Kelvin"By Permission of the Berlin Photographic Co., New York."
Lord Kelvin (Sir William Thomson), (1824-1907). Professor of Physics, Glasgow University. Invented the absolute scale of temperature: also many practical electrical measuring instruments. The foremost physicist of the latter part of the nineteenth century.
Michael Faraday"By Permission of the Berlin Photographic Co., New York."
Michael Faraday (1791-1867). Famous English Physicist. Made many discoveries in electricity and magnetism; "Greatest experimentalist of the nineteenth century."
Fig. 285.?A series-wound dynamo.Fig. 286.?A shunt-wound dynamo.
Fig. 287.?A compound-wound dynamo.
B. The shunt wound dynamo (see Fig. 286) sends a part only of the current produced through the field coils. The latter are of many turns of fine wire so as to use as little current as possible. The greater part of the current goes to the main circuit. If the number of lamps or motors[Pg 334] connected to the main circuit is increased, the voltage is lessened which weakens the current in the field coils, causing a weaker field and still lower voltage, producing a fluctuating E.M.F. which is unsatisfactory for many purposes. This fault is overcome by
C. the compound wound dynamo. This dynamo has both shunt and series coils upon its fields. (See Fig. 287.) If more current is drawn into the main circuit with this dynamo, the series coils produce a stronger field compensating for the weaker field of the shunt coils, so that uniform voltage is maintained. The compound wound generator is therefore the one most commonly employed.
Important Topics
1. Laws of electromagnetic induction (a) conditions, (b) E.M.F., (c) direction.2. Devices, (a) magneto, (b) dynamo: series, shunt, compound.
3. Illustrations of the laws.
Exercises
1. Under what conditions may an electric current be produced by a magnet?2. Show how Lenz's Law, follows from the principle of conservation of energy.
3. A bar magnet is fixed upright with its north-seeking pole upward. A coil is thrust down over the magnet. What is the direction of the current induced in the coil? Explain.
4. In what two ways may a current be induced in a closed coil?
5. What method is employed in the magneto? In the dynamo?
6. What is the nature of the current produced in the armature coil of a magneto, that is, is it direct or alternating? Why?
7. What is the resistance of a 20-watt tungsten lamp if the E.M.F. is 115 volts?
8. Find the resistance of a 40-watt tungsten lamp when the voltage is 115? How much heat will it produce per minute?
9. An Edison storage battery cell on a test gave a discharge of 30 amperes. The average voltage was 1.19. What was the resistance of the cell?
[Pg 335]
10. Eight storage cells are connected in series. Each has an E.M.F. of 1.2 volts and an internal resistance of 0.03 ohms. What will be the current flowing through a voltmeter having 500 ohms resistance in circuit with them?
(2) The Dynamo and the Motor
302. The Dynamo may be defined as a machine for transforming mechanical energy into the energy of electric currents by electromagnetic induction. Although electromagnetic induction was discovered in 1821, practical dynamos were not built for about 40 years or until between 1860 and 1870. The great development in the production and use of electric currents has come since the latter date. The principle parts of the dynamo are (a) the field magnet, (b) the armature, (c) the commutator or collecting rings, (d) the brushes. Fig. 288 shows several common methods of arranging the field coils and the armature. Fig. 288.?Several methods of arranging the field coils and the armature of a dynamo. Fig. 289.?A drum armature. Fig. 290.?Armature connected to slip rings producing an alternating current. 304. The Commutator.?For a dynamo to deliver a direct current it must carry upon the shaft of the armature a commutator. The commutator is used to reverse the connections of the ends of the armature coils at the instant that the current changes its direction in the armature. This reversal of connection when the direction of current changes, keeps the current in the outside circuit flowing in the same direction. Fig. 291 is a diagram of an armature with a commutator. The commutator is a split ring, having as many parts or segments as there are coils upon the armature. The brushes touch opposite points upon the commutator as they slide over the surface of the latter. Suppose that the armature viewed from the commutator end rotates in a counter-clockwise direction, also that the currents from the upper part move toward the commutator and out the top brush.
Fig. 291.?The armature coils are connected to a commutator producing a direct current. Fig. 292.?Graphic representation of (1) an alternating current; (2) a pulsating current; (3) a continuous current. Fig. 293.?DeLaval multi-stage turbine and gear driving 750-kw., 750-r.p.m., 600-volt direct-current generator. The current represented in Fig. 292 (2) is called a pulsating current.
Fig. 294.?A wire carrying a current across a magnetic field is pushed sideways by the field. Set up two bar electromagnets with unlike poles facing each other about an inch apart. A wire connected to a source of current is hung loosely between the poles as in Fig. 294. The circuit through the wire should contain a key or switch. If a current is sent through the electromagnets and then another is sent through the wire, the[Pg 340] latter will be found to be pushed either up or down, while if the current is reversed through the wire it is pushed in the opposite direction. These results may be explained as follows:
Consider the magnetic field about a wire carrying a current (See Fig. 295.) If such a wire is placed in the magnetic field between two opposite poles of an electromagnet (Fig. 296), the wire will be moved either up or down. The reason for this is shown by the diagram in Fig. 297. Here a wire carrying a current and therefore surrounded by a magnetic field passes across another magnetic field. The two fields affect each other causing a crowding of the force lines either above or below the wire. The wire at once tends to move sideways across the field away from the crowded side. In the figure, the wire tends to move downward.
Fig. 295.?The magnetic field about a wire carrying a current.Fig. 296.?The magnetic field between two unlike poles.
Fig. 297.?The crowding of the lines of force above the wire, pushes it downward. Fig. 298.?The crowding of the lines of force causes the armature to revolve in a clockwise direction. Fig. 299.?View of a one-half horse-power motor. The motorman of a street car reverses the motion of his car by reversing the direction of the current in the armature of the motor.
Fig. 300.?The frame and electromagnet (at left), front bracket and brush holder (at right) of the motor shown in Fig. 299.Fig. 301.?The armature of a motor.
Important Topics
1. The dynamo, four essential parts, action (a) for alternating currents, (b) for direct currents.2. The electric motor: (a) essential parts, (b) action.
Exercises
1. Why is an alternating current produced in the armature of a dynamo?2. How is this current produced? Give careful explanations.
3. What is the result of Lenz's law as applied to the dynamo?
4. Apply the first two laws of electromagnetic induction to the dynamo.
5. What is the power of a dynamo if it produces 40 amperes of current at 110 volts?
6. How much power must be applied to this dynamo if its efficiency is 90 per cent.?
7. A motor takes 10 amperes of current at 220 volts; what is the power of the current in watts? If this motor has an efficiency of 95 per cent., how many horse-power of mechanical energy can it develop?
8. Explain why reversing the current in the armature of a motor reverses the direction of rotation.
9. Find the cost of running a washing machine using a 1/2-horsepower motor 2 hours if the cost of the electricity is 10 cents a kilowatt hour.
10. A 1/8-horse-power motor is used to run a sewing machine. If used for 3 hours what will be the cost at 11 cents a kilowatt hour?
(3) The Induction Coil and the Transformer
307. The Induction Coil.?Practically all electric currents are produced either by voltaic cells or by dynamos. It is frequently found, however, that it is desirable to[Pg 344] change the E.M.F. of the current used, either for purposes of effectiveness, convenience, or economy. The induction coil and the transformer, devices for changing the E.M.F. of electric currents, are therefore in common use. The induction coil (see Fig. 302) consists of a primary coil of coarse wire P (Fig. 303) wound upon a core of soft iron wire, and a secondary coil, S, of several thousand turns of fine wire. In circuit with the primary coil is a battery, B, and a current interrupter, K, which works like the interrupter upon an electric bell. The ends of the secondary coil are brought to binding posts or spark points as at D. Fig. 302.?An induction coil. Fig. 303.?Diagram showing the parts of an induction coil. Fig. 304.?The transformer has a closed core; the induction coil, an open core. Fig. 305.?The laminated iron core of a transformer.Fig. 306.?Cross-section of the transformer shown in Fig. 305 showing the magnetic field around the primary and secondary coils.
Fig. 307.?A commercial transformer. 311. The mercury arc rectifier is a device for changing an alternating current into a direct current. It is frequently used for charging storage batteries where only alternating current is supplied by the electric power company.[Pg 348] It consists of an exhausted bulb containing two carbon or graphite electrodes marked G in Fig. 308 and a mercury electrode marked M. It is found that current will pass through such a bulb only from the graphite to the mercury but not in the reverse direction. In operating the device, the secondary terminals of an alternating current transformer T are connected to the graphite terminals of the rectifier. A wire connected to the center of the secondary of the transformer at C is attached to the negative terminal of the storage battery SB. The positive terminal of the battery is connected to the mercury electrode of the rectifier tube through a reactance or choke coil R. This coil serves to sustain the arc between the alternations. Sw is a starting switch, used only in striking the arc. It is opened immediately after the tube begins to glow.
Fig. 308.?Diagram of a mercury arc rectifier. Important Topics
Transformer, induction coil, mercury arc rectifier, construction, action; uses of each.Exercises
1. Does the spark of an induction coil occur at "make" or at "break?" Why?2. What must be the relative number of turns upon the primary and secondary coils of a transformer if it receives current at[Pg 349] 220 volts and delivers current at 110? Also show by diagram.
3. Would the transformer work upon a direct current? Why?
4. Explain why the interrupter is a necessary part of the induction coil and not of the transformer.
5. If a building used eighty 110-volt incandescent lamps, what would be necessary to light them if they were joined in series? Why would this not be practical?
6. If a 16-candle-power lamp requires 0.5 ampere upon a 110-volt circuit what current and voltage will be needed to operate 12 such lamps in parallel?
7. What will it cost to run these lamps 4 hours a night for 30 days at 10 cents per kilowatt hour?
8. If a mercury arc rectifier uses 5 amperes of current at 110 volts alternating current to produce 5 amperes of direct current at 70 volts, what is the efficiency of the rectifier?
9. Compute the heat produced in a 40 watt tungsten lamp in 1 minute.
10. Compute the heat produced in a 60 watt carbon incandescent lamp in 1 hour.
(4) The Telephone
312. The Electric Telephone.?This is an instrument for reproducing the human voice at a distance by an electric current. The modern electric telephone consists of at least four distinct parts (see Fig. 312); viz., a transmitter, an induction coil, an electric battery, and a receiver. The first three of these are concerned in sending, or transmitting over the connecting wires a fluctuating electric current, which has been modified by the waves of a human voice. The receiver, is affected by the fluctuating current and reproduces the voice. It will be considered first, in our study.313. The telephone receiver was invented in 1876 by Alexander Graham Bell. It consists of a permanent steel magnet, U shaped, with a coil of fine insulated copper wire about each pole. (See Fig. 310.) A disc of thin sheet iron is supported so that its center does not quite touch the[Pg 350] poles of the magnet. A hard rubber cap or ear piece with an opening at its center is screwed on so as to hold the iron disc firmly in place.
Fig. 309.?The simplest telephone system. It consists of two telephone receivers connected in series on a circuit. It will work, but not satisfactorily. Fig. 310.?A telephone receiver. This receiver has a permanent horseshoe magnet with a coil about each pole. Fig. 311.?A telephone transmitter.Fig. 312.?Telephone instruments at one end of a talking circuit.
316. A complete telephone system operating with a local battery is shown in Fig. 312. A person speaking into the transmitter causes a fluctuation in the electric current in the transmitter as described in Art. 315. This fluctuating current passes through the primary coil of the induction coil Ic. This fluctuating current produces a fluctuating magnetic field in its core. This fluctuating field induces an alternating current in the secondary coil which alternates just as the primary current fluctuates,[Pg 352] but with a much higher E.M.F. than the latter. The alternating current passes to the receiver which reproduces the speech as described in Art. 313. The line circuit includes the secondary of the induction coil, the receiving instrument and the receiver of the sending instrument so that the voice is reproduced in both receivers. An electric bell is placed at each station to call the attention of parties wanted. The movement of the receiver hook when the receiver is lifted, disconnects the bell and closes the talking circuit. The latter is opened and the bell connected when the receiver is hung up again.
Fig. 313.?Diagram of a telephone system as used in a large exchange. Important Topics
1. Receiver: parts, action.2. Transmitter: parts, action.
3. Induction coil, bell, line wires, etc.
4. Action of the whole device.
Exercises
1. State three important electrical laws or principles that are employed in the operation of the telephone. What is the application of each?[Pg 353]
2. Connect the binding posts of a telephone receiver with a sensitive galvanometer and press on the diaphragm of the receiver; a deflection of the galvanometer will be noticed. Release the diaphragm and a reflection in the opposite direction is seen. Explain.
3. Is the current passing through the transmitter the one going to the receiver of the instrument? Explain.
4. Does the receiver at the telephone used by a person repeat the speech of the person? Explain.
5. How many 0.5 ampere lamps can be used with a 6 ampere fuse?
6. Why is it necessary to have a rheostat connected in series with a stereopticon or moving picture machine while a rheostat is not used with arc lights out doors?
7. How many candle power should a 60 watt carbon incandescent lamp give, if its efficiency is 3.4 watts per candle power?
8. Three incandescent lamps having resistances of 100, 150, and 240 ohms, respectively, are connected in parallel. What is their combined resistance?
Review Outline: Induced Currents
Induced currents; 3 laws, illustrations.Construction, action, and uses of?magneto, dynamo, induction coil, transformer, motor, telephone. Mercury arc rectifier.
Terms?primary, secondary, for coils and currents, armature, commutator, slip ring, brush, rectifier, open core, series, shunt, and compound connections for dynamos.
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