=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. [Illustration: 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. [Illustration: 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. [Illustration: FIG. 415.--A coherer.]