HEliTZ's EXPERIMENTS. 215 



slioit time, but then each oscilhitioii takes an exeeedingiy short time. To 

 .net some idea of what (luaiitities we are dealing- \vith,eousider the rates 

 ol' oscilhition which wonhl give wave-lengths that were short enough to 

 be conveniently dealt with in laboratories. Three liundred million per 

 second would give us waves 1 meter long; consider what is mcaut by 

 1(K),0U0,U(H) i)er second. Wemayget some conception of it by calculating 

 the time corresponding to one hundred million seconds. It is more than 

 three years and two montlis. The pendulum of a clock would have to 

 oscillate three j^ears and two months before it would have performed 

 as many oscillations as we require to be performed in one second. The 

 ])endulum of a clock left to itself without weights or springs to drive it, 

 and only given a single impulse, would practically cease to vibrate after 

 it had i^erformed 40 or 50 vibrations, unless it were veiy heav}-, /. c., had 

 a great store of energy or were very delicately suspended, and exposed 

 only a small resistance to the air. A light pendulum would be stopped 

 by communicating" motion to the air after a very few vibrations. The case 

 of a Leyden Jar discharge is more like the case of a nmss on a spring 

 than the case of a pendulum, because in tlie cases of the Leyden Jar 

 there is nothing quite analogous to the way in which the earth pulls 

 the i>e.ndulum : it is the elasticity of the aether that causes the»electric 

 currents in the Leyden jar discharge, Just as it is the elasticity of the 

 spring that causes the motion of tlie matter attached to it in the cas(^ 

 of a mass vibrating on a spring. 



It is possible to push this analogy still furthei. Under wiuit condi- 

 tions would the spring vibiate most raj>idly !? When the s])rihg was 

 stiff and the mass small. What is meant by a spring" being stiff? When 

 a considerable force only bends it a little. This corres])onds to a con- 

 siderable electric force only electrifying the Leyden Jar coatings a little, 

 i. c. to the Leyden jar having" a small capacity. We would conse(}uently 

 expect that the discharge of a Leyden Jar with a small capacity would 

 vibrate more rapidly than that of one witli a large c;ii)a<'ity, and this is 

 the case. In order to make a Leyden Jar of very small capacity we nmst 

 have small conducting surfiices as far i-.part as possible, and two sepa- 

 rate [)lates or knobs do very well. The second condition for rapid vibra- 

 tion was that the mass moved siiould l)e small. In the case of electric 

 currents what keeps the current running after the plates have become 

 discharged and re-charges them again, is tlie so-called self-induction of 

 the current. It would be well to look upon it as magnetic energy stored 

 up in the oether around the current, but whatever view is taken of it, it 

 evidently corresponds to the mass moved, whose energy keeps its mov- 

 ing after the spring is unbent, and re-bends the spring again. Hen(!c we 

 may conclude that a small self-iiuluction will favor rapidity of oscillation, 

 and this is the case. To attain this we must make the distance the cur- 

 rent has to run from plate to plate as short as i^ossible. The smaller the 

 plates and the shorter t\ui connecting wiie the nun^e rapid tlie vil)ra- 

 tions; in fact, the rapidity of vibration is dire(;tly proportional to the 



