316 Dr. Sumpner and Mr. Phillips on a 



inductive resistances as indicated by the zigzag lines in the 

 figures, (iii.) the moving coil m.c. o£ the instrument is placed 

 directly across the bridge (using a reversing key when 

 desirable). 



It results from the special properties of" the instrument 

 that the flux in the gap o£ the electromagnet is in quadrature 

 with the applied voltage (and thus in quadrature with A). 

 The inductances or capacities produce voltages or currents 

 also in quadrature with A, and thus in phase with the flux, 

 so that their phase is such that they produce the maximum 

 torque on the moving system. 



These methods have all been thoroughly tested on alternate 

 current circuits with the present instrument, and with most 

 satisfactory results. The balance can be adjusted with ease 

 to one part in 10,000, when the voltages set up on the coils 

 or condensers are merely of the order of one volt, and thus 

 suitable for use with the resistance boxes ordinarily found in 

 laboratories. 



Certain special points call for notice. When a balance of 

 great precision is needed, the minute electromotive force e, 

 induced in the moving coil by the alternating field of the 

 magnet, tends to cause a small deflexion disturbing the 

 balance. When the moving coil circuit is essentially non- 

 inductive, as for the cases of figs. 3, 4 and 5, the current due 

 to e will be in phase with £, and in quadrature with the flux, 

 and in such cases the corresponding deflexion will in general 

 be negligible. For the inductive circuits represented in 

 figs. 2, 6 and 7, this will not be the case, and a small de- 

 flexion due to e will occur. But in all cases any effect due to e 

 can be accurately eliminated by using a false zero method, 

 that is, by adjusting the balance till the reading on the scale 

 is unaltered by switching the bridge current A on or off. 

 The induced voltage e is due to the voltage applied to the 

 field coil, and is unaffected by changes in A. In most 

 cases it will be found sufficiently accurate to take the mean 

 of the two conditions of balance obtained by using a 

 reversing key with the moving coil. The false zero method 

 is simpler and is mathematically accurate, though in prac- 

 tice, as with all false zero methods, there is a liability to 

 a small error due to the variations of the false zero deflexion 

 in sympathy with fluctuations in the main current or 

 voltage. 



The formula given for balance expresses the necessary and 

 sufficient condition that the two electromotive forces set up in 

 the coils, or on the condensers, of the bridge, send, through 

 the moving coil, currents which are equal in magnitude and 



