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V. On a New Continuous-Balance Method of Comparing an 

 Inductance with a Capacity. By John P. D ALTON, ALA., 

 D.Sc* 



§ 1. TT7"HEN alternating current is used for inductance 

 ▼ t determinations by null methods, the condition 

 to be fulfilled in order to obtain a true balance is that the 

 potential difference across the terminals of the indicator — 

 telephone or vibration galvanometer — should vanish at every 

 instant, so that the potential differences across each pair of 

 arms from a common junction to the galvanometer must 

 have the same amplitude and they must also be in the same 

 phase. Of the laboratory methods at present commonly 



employed for determining the ratio —;, Anderson's method 



(Stroud's is essentially the same, being the conjugate arrange- 

 ment) is the only one which fulfils the requirements for 

 continuous balance. The original Maxwell scheme might, 

 perhaps, be added, but it hardly ranks as a practical method 

 on account of its tedious successive approximations. In the 

 Rimington and Pirani methods continuous balance can be 

 obtained for one definite frequency only, depending upon 

 the inductance, capacity, and resistances used ; and, if the 

 current is not simply harmonic, continuous balance is not 

 possible at all. 



Since the advent of the tuned telephone and the vibration 

 galvanometer, a.c. methods have come into more general use, 

 and it is as well to have as many independent methods as 

 possible which give a true balance. The object of the present 

 note is to describe a method of comparison which has proved 

 workable, convenient, and satisfactory, and which deter- 

 mines a continuous balance for all frequencies (that is, of 

 course, as long as the resistance is not a function of the 

 frequency].' 



§ 2. It may be of some assistance to look first into the 

 cause of the imperfectness of the balance given by aggregate 

 methods when tried with alternating current. In the Pirani 

 arrangement, for instance, we have, leaving the galvanometer 

 out of account for the present, two circuits in parallel — the 



* Communicated by the Author (now of S. A. School of Mines, 

 Johannesburg). 



