a ]\\iV('-Tr((cer and Aiuili/ser, 553 



6. In order to either plot or photograph the wave-forin o£ 

 a nia<^netic flux F osciUating in any specimen of iron, a coil 

 of a few turns (v say) of wire is coiled round it, and the ends 

 led to one pair of brushes of the synchronous commutator. 

 From the other pair of brushes the circuit is completed 

 through a high resistance r and a dead-beat galvanometer as 

 before. In this case also for each position of the movable 

 brushes the galvanometer will show a steady deflexion, which 

 is proportional to the instantaneous value of the flux at the 

 instants of commutation. 



As the time-eonstiint of the secondary is inai)preciable, the 

 current in it will be 



r dt ' 



and it can be shown, as in § 2, that if commutation be effected 



T 

 at times ^, ^ + ^ , ^ + T, &c., the galvanometer-deflexion ob- 



tained is the same as would be produced by a steady current 



that is, a constant into the instantaneous value of the flux at 

 the time t. 



If y8 be the galvanometer-deflexion obtiiined corresponding 

 to commutation at t^ &c., and if \ be the reducing factor of 

 the galvanometer as before, then 



bv which the instantaneous value of the flux at time t is 

 given absolutely in terms of the corresponding galvanometer- 

 deflexion. 



7. The following table gives the actual galvanometer- 

 readings taken in an experiment with a ring made of 1358 

 turns of varnished iron wire of '0375 cm. diameter. 



The magnetizing-current was supplied by a rotary trans- 

 former (which also drove the commutator) supplied with 

 direct current from storage-cells. It flowed through a coil 

 of 187 turns wound uniformly in one layer all round the 

 rincr. The M of the air-transformer was '000610 henries ; 

 the number of turns v on the secondary of the ring was 10, 

 and the mean length of the iron circuit 31*81) cms. 



