484 Lord Rayleigh on the Self-induction and 



closeness of approach in the two cases. The readings for re- 

 sistance were sensibly the same, whether the currents were 

 steady (balance being tested by a galvanometer), or were 

 variable with a frequency of 1050 per second. They were 

 also unaffected by the reversal of one of the wires. 



With the same pair of double coils an interesting experiment 

 may be made by observing the effect of closing the second 

 wire upon the apparent resistance and self-induction of the first. 

 To steady currents the resistance of one of the copper wires 

 was 1*75, unaltered by closing the circuit of the other wire. 

 With secondary open, the same resistance was found to apply 

 to periodic currents of frequency 1050 ; but when the 

 secondary was closed the resistance rose to 2*67. On the 

 other hand, the closing of the secondary reduced the self- 

 induction from 11 0, 2 to 4°*7. It will be instructive to com- 

 pare these results with Maxwell's formulae : — 



"-W55- n 



which we may do by means of a value of M (the mutual in- 

 duction) deduced from the previous experiment, in which the 

 wires were connected in series. Thus 



M = ll°-0 = ll-0xl553centim. 

 From the present experiment, 



R=S=1'75 inches of slide = 1-75 X '0492 x io 9 ^? 1 ^; 



sec. 



L = N=11°2 of compensator = 11*2 x 1553 centim., 



P = 2ttx1050; 

 so that 



R'+^L 2 "" ou ' 



Thus, according to the formulae, the resistance R' to the 

 periodic currents should be 



R'=B + '60E = 2-80 inches of slide. 



This compares with the 2*7 actually found. In like manner 



I/ = L — *60L = 4 0, 5 of compensator, 



agreeing as well as could be expected with the observed value 

 4°-7. 



^ Similar experiments were made on the double coil of iron 

 wire. With secondary open, the resistance of one wire to 



