416 Professor Oliver Lodge [March 8, 



skin, as your Professor of Natural Philosophy* has shown, and 

 accordingly it does not get magnetised ; and so far from increasing 

 the inductance of the discharge circuit it positively diminishes it by 

 the reaction effect of these induced currents : it acts, in fact, much as 

 a mass of copper might be expected to do. 



The conditions determining rate of oscillation being understood 

 we have next to consider what regulates the damping out of the 

 vibrations, i. e. the total duration of the discharge. 



Resistance is one thing. To check the oscillations of a vibrating 

 spring you apply to it friction, or make it move in a viscous medium, 

 and its vibrations are sj^eedily damped out. The friction may be 

 made so great that oscillations are entirely prevented, the motion 

 being a mere dead-beat return to the position of equilibrium ; or, 

 again, it may be greater still, and the motion may correspond to a 

 mere leak or slow sliding back, taking hours or days for its 

 accomplishment, With very large condensers, such as are used in 

 telegraphy, this kind of discharge is frequent, but in the case of a 

 Leyden jar discharge it is entirely exceptional. It can be caused by 

 including in the circuit a wet string, or a capillary tube full of 

 distilled water, or a slab of wood, or other atrociously bad conductor 

 of that sort ; but the conditions ordinarily associated with the 

 discharge of a Leyden jar, whether it discharge through a long or a 

 short wire, or simply through its tongs, or whether it overflow its 

 edge or puncture its glass, are such as correspond to oscillations, and 

 not to leak. [DiscLarge jar first through wire and next through 

 wood.] 



When the jar is made to leak through wood or water the 

 discharge is found to be still not steady : it is not oscillatory indeed, 

 but it is intermittent. It occurs in a series of little jerks, as when a 

 thing is made to slide over a resined surface. The reason of this is 

 that the terminals discharge faster than the circuit can supj^ly the 

 electricity, and so the flow is continually stopped and begun again. 



Such a discharge as this, consisting really of a succession of small 

 sparks, may readily aj^peal to the eye as a single flash, but it lacks 

 the noise and violence of the ordinary discharge ; and any kind of 

 moving mirror will easily analyse it into its constituents and show it 

 to be intermittent. [Shake a mirror, or waggle head, or opera-glass.] 



It is pretty safe to say, then, that whenever a jar discharge is 

 not oscillatory it is intermittent, and when not intermittent is 

 oscillatory. There is an intermediate case when it is really dead- 

 beat, but it could only be hit upon with special care, while its occur- 

 rence by accident must be rare. 



So far I have only mentioned resistance or friction as the cause 

 of the dying out of the vibrations ; but there is another cause, and 

 that a most exciting one. 



The vibrations of a reed are damped partly indeed by friction and 



* Lord Kayleigh. 



