Fig. 413.
The experiment just described shows that two electrical circuits can be tuned by adjusting their lengths, just as two tuning forks may be made sympathetic by adjusting their lengths. This fact indicates that the discharge of the Leyden jar is oscillatory, since resonance can plainly not be secured except between bodies having natural periods of vibration. This same fact is also shown by examining the discharge of a Leyden jar as it appears when viewed in a rapidly revolving mirror. (See Fig. 414.) The appearance in the mirror shows that the discharge is made up of a number of sparks, often a dozen or more, vibrating back and forth until they finally come to rest. The time of one vibration varies from one millionth to one hundred millionth of a second, depending on the space between the discharging balls and the size of the jars.
Fig. 414.—Photograph of the oscillatory discharge of a Leyden jar.
The discharge of a Leyden jar or of another condenser sets up ether waves that have the speed of light. Heinrich Hertz in Germany first proved this in 1888. These waves are now known as Hertzian waves. The length of these varies from 3 cm. to several miles, depending upon the size and conditions of the discharging circuit.
Fig. 415.—A coherer.
416. The Coherer.—The coherer is a device for detecting electric waves. It consists of a glass tube with metal filings loosely packed between two metal plugs that fit the tube closely. (See Fig. 415.) These filings offer a high resistance to the passage of an electric current, but when electric waves pass through the filings these cohere and allow a weak current to pass through. This current may be strong enough to operate a relay connected with a sounder or bell that gives audible signals. If the tube be tapped the filings will be disturbed and the resistance again made so high that no current can pass through.
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.