342 Lord Rayleigh's Comparison of Methods for the 



to a battery-current which circulates through a coaxal coil of 

 many turns. The revolving disk is touched near its centre 

 and circumference by two wires. If the circuit were simply 

 closed through a galvanometer, the instrument would indicate 

 the current due to the electromotive force of induction acting 

 against the resistance of the circuit. The electromotive force 

 corresponding to each revolution is the same as would be 

 generated in a single turn of wire coincident with the circum- 

 ference of the disk by the formation or cessation of the battery- 

 current. If this be called 7, and M be the coefficient of induc- 

 tion between the coil and the circumeference, m the number 

 of revolutions per second, the electromotive force is mMy. 

 For the present purpose, however, the circuit is not simply 

 closed, but its terminals are connected with the extremities of 

 a resistance R through which the battery-current flows, and 

 the variable quantities are so adjusted that the electromotive 

 force R7 exactly balances that of induction. When the gal- 

 vanometer indicates no current, the following relation, inde- 

 pendent, it will be observed, of the magnitude of the battery- 

 current, must be satisfied, 



R=wM; 

 and from this, M being known from the data of construction, 

 the absolute resistance R of the conductor is determined. 



Tt will be seen that this method has pretty close affinity to I. 

 The secondary circuit is here, in a sense, reduced to a single 

 turn, or rather to as many turns as the disk makes revolutions 

 in a time comparable with the time of swing of the ballistic 

 galvanometer ; but the disadvantage of a reduced number of 

 turns is probably more than compensated for by the continuous 

 character of the induced current, which allows of its being 

 brought into direct opposition to that of the battery. During 

 the months from April to August of the present year I have 

 been occupied in carrying out a determination by this method. 

 Space will not permit of a detailed consideration of the various 

 questions which presented themselves; and I must content 

 myself with a brief statement of the procedure, and with such 

 a discussion of the sources of error as will allow a comparison 

 of this method with others. I hope shortly to communicate a 

 detailed paper upon the subject to the Royal Society. 



One of the principal difficulties to be overcome arises from 

 the exceeding smallness of the resistance R, less than ^q B.A. 

 in my experiments. Lorentz employed an actual column of 

 mercury of known dimensions, so that the result is given at 

 once in terms of mercury. I had intended to follow the same 

 course, but, after some trials, came to the conclusion that 

 there would be difficulties in the way of thus obtaining the 



