124 Prof. Gr. C. Foster on a Method of Determining 



average current through the galvanometer vanishes, the last 

 equation becomes 



(M-L) 7 = (M-L)s, 

 which requires either (1) that 7 = 2 (that is, that the primary 

 current has become steady), or (2) that M — L = 0. 



When the last-mentioned condition is satisfied, we ought to 

 be able to replace the galvanometer between the points A and 

 e by a telephone, and to employ a rapid make-and-break *. 



By using a pair of coils of known and invariable coefficient 

 of mutual induction, the experimental process described above 

 may afford a ready way of determining the capacity of a 

 condenser; but for this purpose the method does not appear 

 to possess any advantage over the well-known methods of 

 De Sauty and Sir William Thomson for determining the 

 capacity of condensers by comparison with a known standard, 

 more especially as a known condenser is more frequently at 

 hand than a pair of coils of known coefficient f. 



The limit of accuracy attainable in measurements by the 

 method here described depends essentially upon the sensi- 

 tiveness of the galvanometer employed; but with a given 

 galvanometer, the method is susceptible of various degrees of 

 accuracy according to the relative values given to the two 

 variable resistances p and r. 



These should be so adjusted that, for a given value of the 

 difference Cp*— M, that is, for a given error in the adjustment 

 of the product of the resistances p and r, the quantity of elec- 

 tricity traversing the galvanometer may be as great as possible. 

 This requires that the resistances should fulfil the condition 



£-1 



where r f is the resistance between A and D through the 

 primary coil and battery. Let X and Y stand for the 



* Experiments in this direction, made since the reading of this paper 

 before the Physical Society by Mr. F. Womack, have not yet led to a 

 fully satisfactory result. 



f Since this paper was read, Professor Roiti, of Florence, has kindly 

 sent me a copy of a paper communicated by him to the Royal Academy 

 of Sciences of Turin (Memoirs of the Academy, series ii., vol. xxxviii.), in 

 which he describes a method, very similar to that indicated in the text, 

 for the absolute measurement of the capacity of condensers, using for the 

 purpose a pair of coils whose coefficient of mutual induction is accu- 

 rately known from their dimensions and relative positions. The chief 

 difference between Professor Roiti's arrangement and that given in this 

 paper is that, instead of inserting the galvanometer between the points 

 marked a and e in fig. 3, he includes it in the branch, afe, containing 

 the secondary coil. Between a and e he places a contact-key h, which, 

 as well as the key k, is opened and closed mechanically, the two keys 

 being moved by cranks attached to the same axle at right angles to 

 each other. 



