DYNAMIC MEASUREMENTS ON ELEfTROM \(;\!;tI( Di \i( i;s 1 117 



The dc instrument will read the average vahie of current for the cycle. 

 As the period of the cycle is T, and ti is the chosen delay for .-losure of 

 the B contact measured from the time of the A closure, tlw diK . t (-ur- 

 rent mdicated by the mstrument will be: 



(3) 



{Rn. + Rc)f • 



Now the product LJ at ^i when the B contact is closed must be zero. 

 Furthermore, if the period T is long in comparison with the time of rise 

 and decay of the flux, and with the time constant L/R of the measuring 

 circuit, the product Lmi is also negligible at ^ = T. We then have: 



^t, - ^T = ?^ X 10' Maxwells, (4) 



where R is now the total resistance of the measuring circuit . The factor 

 10^ has been added to convert from practical units to c.g.s. units. The 

 flux $r is the residual flux, so that if this is taken as a reference value, 

 the value of the flux at the time of closing the B contact is simply 



$ = ?:IIl X 10' Maxwells. (5) 



It may be objected that the flux in equation (2) is made up not only of 

 the flux to be measured but also of a component due to the current in 

 the measuring circuit. This is true, and the flux linking the search coil 

 changes differently during the time the B contact is closed compared to 

 that without the instrument circuit. Note, however, that we are not 

 concerned with how the flux varies between the times h and T but only 

 with its value at the limits. Since the flux at ti is that which has been 

 established with the search coil circuit open, the one requirement is that 

 the constants be such that the current in the measuring circuit he sub- 

 stantially zero just before T. This can be verified through the meas- 

 urements themselves by noting whether there is any change in measured 

 flux in this interval. If there is, then the cycle time T can be increased 

 until this requirement is met. 



It will be convenient later to regard the switched meter as a linearized 

 circuit composed of an equivalent dc voltage 



E = ^X 10-* vdis (6) 



in series 



with the resistor R, causing a dc current h to flow, Fig. 1 (g). 



