DYNAMIC MEASUREMENTS ON ELECTROMAGNETIC DEVICES 1445 



by the A contact. The cycle time is chosen for complete operation and 

 the time selector is set for a time near the end of the operate interval. 

 At this time the tested device is known to be at its maximum displace- 

 ment, and the instrument reading, using the displacement connection, 

 corresponds. A convenient instrument indication is obtained by adjust- 

 ment of the amplifier gain controls. For instance if the displacement 

 were 0.040'', an instrument reading of 200 microamperes could be used. 

 At any other time, when the parts are in relative motion, the instantane- 

 ous displacement is read directly from the instrument using the same 

 scale conversion factor. 



The calibration for velocity measurements depends upon the displace- 

 ment calibration and relationship between the input differentiating ca- 

 pacitor and the resistor it replaces. Once a displacement gain setting has 

 been chosen, it must not be altered during the associated velocity meas- 

 urements except by calibrated steps. The differentiating capacitor has 

 been chosen so that an instrument displacement deflection, correspond- 

 ing to a given number of thousandths of an inch, represents the same 

 number in inches per second. For the example given above, a 200-micro- 

 ampere instrument reading would represent an instantaneous velocity of 

 40 inches per second, with a linear calibration for intermediate readings. 



A plot of measured displacement and velocity for a fast wire spring 

 relay shown in Fig. 21, will be described when system errors are con- 

 sidered. 



Photocell and Impedance Transformer Amplifier. A schematic of the 

 photocell and dc impendance transformer is shown in Fig. 17. The high 

 vacuum photocell has an impedance of thousands of megohms and acts 

 essentially as a constant current device, the current depending upon the 

 instantaneous illumination. This current is connected to the grid of a 

 series stabilized^ twin triode amplifier tube, to which grid also is con- 

 nected the current through the feedback resistor and a balancing current 

 from the — 250-volt powder supply. The zero adjustment is made under 

 quiescent conditions for the desired dc output voltage, there being, of 

 course, essentially no dc current in the grid. 



The Western Electric low grid current 420A input tube is stabilized 

 both for heater voltage and plate potential. It provides a voltage ampli- 

 fication of exactly fji/2 or 35, by virtue of the upper tube ha\ing an im- 

 pedance exactly equal to that of the plate of the lower tube. This input 

 tube is mounted immediately behind the photocell in the same shield. 



The output from the first amplifier tube is connected through a double 

 shielded cable to the output tube mounted on the main chassis. The 

 5687 twin triode output tube uses both halves as cathode followers, one 



