Invisible Radiations
INVISIBLE RADIATIONS
(1) Electric Waves and Radio-activity
415. Oscillatory Nature of the Spark from a Leyden Jar.?In studying sound (Art. 339), the sympathetic vibration of two tuning forks having the same rate of vibration was given as an illustration of resonance. The conditions for obtaining electrical resonance by the use of two Leyden jars are given in the following experiment.
Join the two coats of a Leyden jar (Fig. 413) to a loop of wire L, the sliding crosspiece M
being arranged so that the length of the loop may be changed as
desired. Also place a strip of tinfoil in contact with the inner coating
and bring it over to within about a millimeter of the outer coating as
indicated at G. Now join the outer coating of another exactly similar jar A
to a wire loop of fixed length, the end of the loop being separated
from the knob connected to the inner coating, a short distance at P. Place the jars near each other with the wire loops parallel and connect coatings of A to the terminals of a static machine or an induction coil. At each discharge between the knobs at P, a spark will appear in the other jar at G, if the crosspiece M is so adjusted that the areas of the two loops are exactly equal. When the wire M is moved so as to make the areas of the two loops quite unequal, the spark at G disappears.
417. Wireless Telegraphy.?In 1894 Marconi, then a young man of twenty, while making some experiments with electrical discharges discovered that the coherer would detect electrical waves at a considerable distance from their source and that by the use of a telegraph key the "dots and dashes" of the telegraph code could be reproduced by a sounder attached to a relay. At present the coherer is used principally in laboratory apparatus, as much more sensitive detectors are now available for commercial work. The essential parts of a modern wireless telegraph apparatus as used in many commercial stations are shown in Fig. 416.
Alternating current at 110 volts is sent into the primary, P, of a transformer, the secondary, S,
of which produces a potential of 5000 to 20,000 volts. The secondary
charges a condenser until its potential becomes high enough to produce a
discharge across a spark gap, SG. This discharge is oscillatory,
the frequency being at the rate of about one million a second,
depending upon the capacity of the condenser and the induction of the
circuit.
These oscillations pass through the primary of the oscillation transformer, inducing in the secondary, electric oscillations which surge back and forth through the antenn?, or aerial wires, A. These oscillations set up the "wireless waves." The production of these waves is explained as follows: An electric current in a wire sets up a magnetic field spreading out about the conductor; when the current stops the field returns to the conductor and disappears. The oscillations in the antenn?, however, have such a high frequency, of the order of a million a second, that when one surge of electricity sets up a magnetic field, the reverse surge immediately following sets up an opposite magnetic field before the first field can return to the wire. Under these conditions a succession of oppositely directed magnetic fields are produced which move out from the antenn?[Pg 451] with the speed of light and induce electric oscillations in any conductors cut by them.
These oscillations pass through the primary of the oscillation transformer, inducing in the secondary, electric oscillations which surge back and forth through the antenn?, or aerial wires, A. These oscillations set up the "wireless waves." The production of these waves is explained as follows: An electric current in a wire sets up a magnetic field spreading out about the conductor; when the current stops the field returns to the conductor and disappears. The oscillations in the antenn?, however, have such a high frequency, of the order of a million a second, that when one surge of electricity sets up a magnetic field, the reverse surge immediately following sets up an opposite magnetic field before the first field can return to the wire. Under these conditions a succession of oppositely directed magnetic fields are produced which move out from the antenn?[Pg 451] with the speed of light and induce electric oscillations in any conductors cut by them.
While the electric waves are radiated in all directions from the aerial, the length
of the waves set up is approximately four times the combined length of
the aerial wires and the "lead in" connection to the oscillation
transformer.
[Pg 452]
The electric waves induce effective electrical oscillations in the
aerial of the receiving station, even at distances of hundreds of
miles, provided the receiving transformer, RT, is "tuned" in resonance with the transmitting apparatus by adjustments of the variable condenser, VC, and the loading coil, L. The detector of these oscillations in the receiving transformer is simply a crystal of silicon or carborundum, D, in series with two telephone receivers, Ph.
The crystal detector permits the electric oscillations to pass through
it in one direction only. If the crystal did not possess this property,
the telephone could not be used as a receiver as it cannot respond to
high frequency oscillations. While one spark passes at SG, an intermittent current passes through the receiver in one direction. Since some 300 to 1200 sparks pass each second at SG while the key, K, is closed, the operator at Ph hears a musical note of this frequency as long as K is depressed. Short and long tones then correspond to the dots and dashes of ordinary telegraphy. In order to maintain a uniform tone a rotary spark gap, as shown, is often used. This insures a tone of fixed pitch by making uniform the rate of producing sparks.
The Continental instead of the Morse code of signals is generally employed in wireless telegraphy, since the former employs only dots and dashes. The latter code employs, in addition to dots and dashes, spaces
which have sometimes caused confusion in receiving wireless messages.
The United States government has adopted the regulations of the International Radio Congress
which directs that commercial companies shall use wave lengths between
300 and 600 or above 1600 meters. Amateurs may use wave lengths less
than 200 meters and no others, while the government reserves the right
to wave lengths of 600 to 1600 meters. See p. 459 for Continental
telegraph code.418. Discharges in Rarefied Air.?Fig. 417 represents a glass tube 60 or more centimeters long, attached to an air pump. Connect the ends of the tube to the terminals of a static machine or of an induction coil, a-b. At first no sparks will pass between a and f, because of the high[Pg 453] resistance of the air in the tube. Upon exhausting the air in the tube, however, the discharge begins to pass through it instead of between a and b. This shows that an electrical discharge will pass more readily through a partial vacuum than through air at ordinary pressure. As the air becomes more and more exhausted, the character of the discharge changes. At first it is a faint spark, gradually changing until it becomes a glow extending from one terminal to the other and nearly filling the tube.
422. Radio-activity.?In 1896 Henri Becquerel of Paris discovered that uranium and its compounds emit a form of radiation that produces an effect upon a photographic plate that is similar to that resulting from the action of "X" rays. These rays are often called Becquerel rays in[Pg 457] honor of their discoverer. The property of emitting such rays is called radio-activity, and the substances producing them are called radio-active.
In 1898, Professor and Mme. Curie after an investigation of all the elements found that thorium, one of the chief constituents of incandescent gas mantles, together with its compounds, was also radio-active. This may be shown by the following experiment:
Place a flattened gas mantle upon a photographic plate and leave
in a light tight-box for several days. Upon developing the plate in the
usual way a distinct image of the mantle will be found upon the plate.
423. Radium.?Mme. Curie discovered also that pitch-blende
possessed much greater radio-active power than either thorium or
uranium. After prolonged chemical experiments she obtained from several
tons of the ore a few milligrams of a substance more than a million
times as active as thorium or uranium. She called this new substance radium.
Radium is continually being decomposed, this decomposition being
accompanied by the production of a great deal of heat. It has been
calculated that it will take about 300 years for a particle of radium to
be entirely decomposed and separated into other substances. It is also
believed that radium itself is the product of the decomposition of
uranium, atomic weight 238, and that the final product of successive
decompositions may be some inert metal, like lead, atomic weight 207.The radiation given off by radio-active substances consists of three kinds: (A) Positively charged particles of helium called alpha rays: (B) negatively charged particles called beta rays: (C) gamma rays.
The alpha rays have little penetrating power, a sheet[Pg 458] of paper or a sheet of aluminum 0.05 mm. stopping them. Upon losing their charges they become atoms of helium. Their velocity is about 1/10 of that of light or 18,000 miles a second. The spinthariscope is a little instrument devised by Sir Williams Crookes in 1903 to show direct evidence that particles are continually being shot off from radium. In this instrument (Fig. 424), a speck of radium R is placed on the under side of a wire placed a few millimeters above a screen S covered with crystals of zinc sulphide. Looking in the dark at this screen through the lens L, a continuous succession of sparks is seen like a swarm of fireflies on a warm summer night. Each flash is due to an alpha particle striking the screen. The beta rays are supposed to be cathode rays or electrons with velocities of from 40,000 to 170,000 miles a second. The gamma rays are supposed to be "X" rays produced by the beta rays striking solid objects.
Important Topics
1. Oscillatory nature of discharge of Leyden jar. Proofs.2. Wireless telegraphy and telephony.
3. Electrical discharges in rarefied gases.
[Pg 459]
4. Cathode and "X" rays.
5. Electromagnetic theory of light.
6. Radio activity and radium.
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