470 Lord Rayleigh on the Self-induction and 



are certain other points raised by him, or suggested by his 

 work, which seem worthy of consideration ; and I propose in 

 the present paper to give an account of some investigations, 

 mainly experimental, carried on during the summer months, 

 which may, I hope, tend to settle some controverted questions. 

 Prof. Hughes's first apparatus consists of a Wheatstone's 

 quadrilateral, with a telephone in the bridge, one of the sides 

 of the quadrilateral being the wire or coil under examination, 

 and the other three being the parts into which a single 

 German-silver wire is divided by two sliding contacts. If the 

 battery-branch be closed, and a suitable interrupter be intro- 

 duced into the telephone-branch, balance may be obtained by 

 shifting the contacts. Provided that the interrupter introduces 

 no electromotive force of its own*, the balance indicates the 

 proportionality of the four resistances. If P be the unknown 

 resistance of the conductor under test, Q, R the resistances of 

 the adjacent parts of the divided wire, 8 that of the opposite 

 part (between the sliding contacts), then, by the ordinary rule, 

 PS=QR; while Q, R, S are subject to the relation 



Q + R+S = W, 



W being a constant. If now the interrupter be transferred 

 from the telephone to the battery-branch, the balance is usu- 

 ally disturbed on account of induction, and cannot be restored 

 by any mere shifting of the contacts. In order to compensate 

 the induction, another influence of the same kind must be 

 introduced. It is here that the peculiarity of the apparatus 

 lies. A coil is inserted in the battery and another in the 

 telephone-branch, which act inductively upon one another, 

 and are so mounted that the effect may be readily varied. 

 The two coils may be concentric and relatively movable about 

 the common diameter. In this case the action vanishes when 

 the planes are perpendicular. If one coil be very much 

 smaller than the other, the coefficient of mutual induction M 

 is proportional to the cosine of the angle between the planes. 

 By means of the two adjustments, the sliding contact and the 

 rotating coil, it is usually possible to obtain a fair silence. 



In his address Prof. Hughes interpreted his observations on 

 the basis of an assumption that the self-induction of P was 

 represented by M, irrespective of resistance, and that the 

 resistance to variable currents could (as in the case of steady 

 currents) be equated to QR/S. In the discussion which fol- 

 lowed I pointed out that this was by no means generally true, 



* This condition is not always satisfied. With the reed-interrupter 

 (see below) a loud sound may sometimes be heard, although the batterv- 

 branch be open. 



