DYNAMIC MEASUREMENTS ON ELECTROMAONKTIC DEVICh^S 1427 



easily be removed by reactance elements added to the bridge as Hhown. 

 It will be noted that the bridge is still of constant resistance, and is still 

 balanced at all frequencies. 



Three adjustments are provided on the amplifier: one controls the 

 feedback, a second the phase correction, and the third the plate current. 

 The amount of suppression is controlled by a three position key which 

 may be used to cut down the sizes of the capacitors in the circuit in a 

 ratio of two and four to one. In cases where one of the higher pulse rates 

 is used, adequate suppression and somewhat faster readings may Ik» 

 secured by using smaller capacitors. With the maximum suppression, 

 two cycles may be reduced to such an extent that it has no detectable 



S 



FREQUENCY 



FREQUENCY »• 



Fig. 7 — Illustration of method for phase compensation of amplifier. 



effect on the meter pointer. Accurate measurements at one cycle may be 

 made, although there is a slight motion of the pointer. 



Analytical studies have indicated that the transient response of the 

 circuit depends to a large extent on the ratio of the interstage coupling 

 capacitor to the output capacitor and moreover that there is an optimum 

 ratio of the phase compensating capacitor to the other two. In altering 

 the amount of suppression therefore, the ratios of the three capacitors 

 are held constant. 



The Switching Circuit 



Requirements. The basic feature of this measuring system is the switch- 

 ing circuit, consisting of the A, B and C contacts. The re(iuirements for 

 these contacts are: (a) Negligible dc resistance, (b) No contact chatter, 

 (c) Contact potential less than 10 microvolts, and (d) Stability of opera- 

 tion of 20 microseconds. Consideration of these facters led to the selec- 

 tion of mercury contact relays as the basic switching element.s. The choice 



