Electric Waves along Wires. 659 



which divided the circuits so that their equivalent lengths 

 were approximately in the ratio of odd to odd numbers. In 

 that case the two ends of a circuit were in opposite electrical 

 conditions, and therefore the length of each circuit was an 

 odd number of half wave-lengths ; but with the closed end 

 KK' the length of the whole circuit BB'KK' must be that of 

 an even number of half wave-lengths, and resonance between 

 the two circuits occurs only when their equivalent lengths 

 contain an odd and an even number of half wave-lengths 

 respectively, which is only possible when these lengths are in 

 the ratios of odd to even numbers. 



When the two circuits are tuned so that their lengths are 

 in the ratio 1 : 2, SBB'S' (giving equivalent lengths for 

 capacities, &c.) is half a wave-length, and the total length of 

 the circuit KBB'K' a complete wave-length *. 



]f now the circuits are entirely separated by making BB' a 

 terminal bridge of the first circuit and placing a similar 

 bridge BiB/ very close to BB', while the parallel wires are 

 cut between the two, there is no conducting connexion 

 between them, and the oscillations in BiKK'B/ are caused 

 entirely by induction f . 



In this form resonance occurs as before without the inter- 

 ference from other systems of waves, the oscillations having 

 a maximum intensity when the two circuits SBB'S' and 

 KBiB/K' are tuned. 



This arrangement, as shown in fig. 4, was used in the 

 following determinations. The wire used for the primary 

 circuit had a diameter *076 cm. and was supported by light 

 ebonite pegs at A, B, B', and A 7 . Similar pegs also held the 

 ends BxB/ of the secondary circuit in position just beyond 

 sparking distance from BB', while the bridge KK 7 with the 

 loop and detector-needle were movable along the parallel 

 wires B } Z and B/Z', whose ends were held in position by silk 

 threads attached to Z and Z\ 



To compare the velocities of the waves along different 

 wires, the obvious method was to keep the frequency constant 

 and to measure the wave-length corresponding to that fre- 

 quency along the various wires. First the arrangement 

 shown in fig. 1 was set up and the position of the bridge, at 

 which resonance between the two circuits occurred when 

 their equivalent lengths were in the ratio 1 : 2, was noted. 



* See Rutherford's paper, Phil. Trans. 1897. 



t Lecher showed that if a double bridge was used and the parallel 

 wires were cut between the ends of the bridge, the intensity of oscillation 

 round the circuit KBB'K' wa3 very slightly changed, showing the 

 oscillations in that circuit to be due to induction. 



