THE BALLISTIC GALVANOMETER 



119 



Consequently 

 R 



S 



By (21) 



+ - 0082 cos 0.422] = 0.9597 

 .0082 s j n 0.422] = 0.2064 

 "+~g = 0.982 



o.oon = L0 04. 



6, 0.982 

 6\ == 1.004 



0.978. 



The error is therefore 2.2 per cent, 

 above conditions gave 



= 0.978. 



An actual test under the 



When the ballistic galvanometer is used in series with a test 

 coil for determining magnetic fluxes through iron cores it is not 

 possible to apply the above 

 purely analytical method, for 

 the law connecting time and 

 the e.m.f. induced in the test 

 coil is not known. In this case 

 the integration must be made 

 with the aid of a planimeter. 



The graph connecting e and 

 t may be obtained by an os- 

 cillograph, an exploring coil 

 being wound for that purpose 

 on the specimen under test. 

 In general, the oscillograph is 

 not inserted in the galvanom- 

 eter circuit for the 



2 



6 8 10 12 14 

 Time. Seconds 



18 20 



FIG. 55. Pertaining to effect of 

 current prolonged discharge through a ballistic 



in it is modified by the e.m.f. galval 

 set up by the movable coil. In cases where this e.m.f. is in- 

 significant compared with that due to the change of flux through 

 the c'rcuit, the separate exploring coil is not necessary. Suppose 

 the e.m.f .-time curve OABC, shown in Fig. 55, has been obtained. 

 The curves OEC and OFC are obtained by multiplying the 

 ordinates of OABC by the corresponding values of e* cos bt 

 and at sin bt, 



