DETERMINATION OF WAVE FORM 629 



The direct-current instrument gives the mean value while the 

 electrodynamometer gives the root-mean-square value. The 

 deflections are controlled by non-reactive resistances, the values 

 of which need not be known. With the commutator running 

 and the brushes adjusted for reversal at the zero point of the 

 wave by the method just given, both instruments are read. 

 Call the dynamometer reading D, and the galvanometer reading 

 G; then 



Effective ** ' Average 



With the commutator stationary a direct-current P.D. is now 

 applied. Then, if D r and G' are the readings, 



' 



and the form factor is 



P.D. effective DG 



" 



P.D. average = D'G 



The Oscillograph. 5 All of the methods for obtaining wave 

 form that have been given require that the phenomena be periodic 

 and that the circuit conditions remain fixed for a considerable 

 time. Also with some of the methods much time must be spent 

 in calculating and plotting the results. Cases are continually 

 arising where it is desirable to investigate phenomena which are 

 transient in character, as for example, that illustrated in Fig. 394. 



0.01 Second 



FIQ. 394,-Showing variation of current and voltage during short-circuit 

 test of 100-ampere enclosed fuse. 



