of Lightning- Conductors. 227 



This diagram shows the arrangement. 



B 



The jar discharges at A in the ordinary way, and simulta- 

 neously a longer spark is observed to pass at B at the far end 

 of two long leads. Or if the B ends of the wire are too far 

 apart to allow of a spark, the wires glow and spit off brushes 

 every time a discharge occurs at A. 



The theory of the effect seems to be that oscillations occur 

 in the A circuit according to equation (3') with a period 

 T = 2ttV(LS) ? 



where L is the inductance of the A circuit, and S is the ca- 

 pacity of the jar. These oscillations disturb the surrounding 

 medium and send out radiations, of the precise nature of 

 light, whose wave-length is obtainable by multiplying the 

 above period by the velocity of propagation. 

 This velocity is knowm to be 



1 



so the wave-length is 



,=,T = 2 7 r x /{^.|} (15) 



Now — is the electromagnetic measure of inductance, and 



S 



^ the electrostatic measure of capacity. Each of these quan- 

 tities is of the dimension of a length, and the wave-length of 

 the radiation is 27T times their geometric mean. 



The propagation of these oscillatory disturbances along the 

 wires towards B goes on according to the following laws : — 



Let l x be the inductance per unit length of the wires ; let s x 

 be their capacity, or permittance as Mr. Heaviside calls it, 

 per unit length ; and let r x be their resistance per unit length. 



Then, for the slope of potential along them, we have 



-£-*"+** oo 



and for the accumulation of charge, or rise of potential with 

 time, 



_^ = I.f (17) 



dt 8 t dx v ' 



Q2 



