Galvanometer for Alternate Current Circuits. 317 



exactly opposite in phase*. But if this condition is not quite 

 fulfilled, the unbalanced current will not necessarily be in the 

 best phase to influence the deflexion unless certain limitations 

 are borne in mind. The resistances R must not be made too 

 small, and the resistances S must not be made too large. 

 Otherwise the sensitiveness of the instrument to indicate want 

 of balance is adversely affected, although the condition of 

 balance remains as stated, except for minute correction terms, 

 due to secondary effects of self-induction, &c, which have 

 been neglected. 



AVe have found on investigation that the only cases which 

 need be considered are those in which condensers are used 

 (figs. 2, 6 and 7). The value of KS/? (where p is 2jt times 

 the frequency) represents tlie tangent of the angle by which 

 the phase of the moving coil current differs from that of the 

 magnet field. It may easily become comparable with unity, 

 as will be apparent from the fact that on 50 cycle circuits 

 with K equal to 1 microfarad, and S equal to 1000 ohms, the 

 value of KS/> is 0*314. But it will be found easy to adjust 

 the conditions of the bridge in all the cases considered so as 

 to render these tests quite satisfactory in practice. 



One or two examples of these bridge methods may be given 

 to illustrate the conditions of working. 



The method of fig. 6 was used to test the values of M for 

 a primary coil of 500 turns in conjunction with two secondary 

 coils. The three coils were wound on a wooden bobbin and 

 the primary wire was suitable for a curivnt of 2 amperes. A 

 current of l'l ampere was passed through the bridge, and 

 2S volts were applied to the 4000 turn coil, the frequency 

 being 50 cycles per second. A standard resistance of 

 0*9995 ohms was used for S, and a standard mica condenser 

 of 1*0155 m.f. was used for K. An ordinary resistance-box 

 was used for R. Using the first secondary coil the value of 

 R 4- r was adjusted to 4225*1 ohms. The corresponding value 

 of M works out to be 4*2885 millihenries. A similar test 

 with the other secondary yielded 3*8499 m.h., and one with 

 the two secondaries in series yielded 8*1398 m.h. The sum 

 of the values of M for the two secondaries is 8*13*4 m.h. 

 The small inconsistency is easily attributable to errors in the 

 resistance-box, to small capacity effects in these resistances, 



* This is strictly true for the cases of figs. 2, 4, and 6. In the cases 

 of tigs. 3,5, and 7 an additional current through the moving coil is caused 

 bv the resistance of the inductance coils. This current is in quadrature 

 with the field, and does not cause any deflexion. There is thus no need 

 for a troublesome double adjustment as in corresponding tests in which 

 other instruments are used. 



