414 



Professor Oliver Lodge 



[March 8, 



The cord is to represent electricity ; the beads represent successive 

 strata in the thickness of the glass of the jar, or, if you like, atoms 

 of dielectric or insulating matter. Extra tension in the cord 

 represents negative potential, while a less tension (the nearest 

 analoo^ue to pressure adapted to the circumstances) represents positive 



Mechanical analogy of a circuit partly dielectric ; for instance, of a charged 

 Cundenser. A is its positive coat, B its negative. 



potential. Forces applied to move the cord, such as winches or 

 weights, are electromotive forces; a clamp or fixed obstruction 

 represents a rheostat or contact-breaker ; and an excess or defect of 

 cord between two strata of matter represents a positive or a negative 

 charge. 



The act of charging a jar is now quite easily depicted as shown in 

 the diagram. 



To discharge the jar one must remove the charging E.M.F. and 

 unclamp the screw, i.e. close the circuit. The stress in the elastic 

 threads will then rapidly drive the cord back, the inertia of the 

 beads will cause it to overshoot the mark, and for an instant the jar 

 will possess an inverse charge. Back again the cord swings, however, 

 and a charge of same sign as at first, but of rather less magnitude, 

 would be found in the jar, if the operation were now suspended. If it 

 be allowed to go on, the oscillations gradually subside, and in a short 

 time everything is quiescent, and the jar is completely discharged. 



All this occurs in the Leyden jar, and the whole series of oscilla- 

 tions, accompanied by periodic reversal and re-reversal of the charges 

 of the jar, is all accomplished in the incredibly short space of time 

 occupied by a spark. 



Consider now what the rate of oscillation depends on. Manifestly 

 on the elasticity of the threads and on the inertia of the matter which 



