1422 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



becomes a differentiator, in which the output current is proportional t 

 rate of change of input voltage. With this one change, the instrumen 

 reads the instantaneous velocity of the armature. Under these condition; 

 the second amplifier differentiates the input voltage once, the outpu 

 transformer differentiates a second time, and the dc instrument integrate 

 once, leaving a net result of one differentiation. 



Description of System Components 

 fluxmeter 

 General 



In the following descriptions of the several components of the systen 

 the general characteristics required are considered, the specific devig 

 tions from ideal are determined, and an evaluation of the measuremen 

 errors is made. The description starts with the dc instrument followe 

 by the associated vacuum tube filter. Then the heart of the system, th 

 contact switching circuit itself is considered. Following this is the timin 

 impulse generating circuit, the counting rings, the time selector, coinci 

 dence circuits, memory, and relay control circuits. These elements mak 

 up the fluxmeter proper. 



The auxiliary circuits for displacement-time and velocity-time meas 

 urements are the concern of the next part of the paper. The component 

 are the optical system, the photocell amplifier, the amplifier-differentiate 

 and finally the output amplifier. 



The last section first shows typical measurements on a telephone typ 

 relay. Then a description of a more fundamental study of dynamic flu 

 rise and decay in solid core electromagnets is given. This study has le^ 

 to two new first approximation equations for dynamic flux rise and deca> 

 as will be seen. 



Dc Instrument and Effect of Damping Resistance 



The dc instrument used for a majority of the measurements is 

 heavily damped 50.0-ohm, 200-microampere full scale instrument, wii 

 a }/2. P^r cent of full scale accuracy. The open circuit decay time constan 

 is about 2 seconds, and with a shorted winding about 4 seconds. On Fi^ 

 5 is shown a plot of the decay time constant referred to a short circuit 

 versus the damping resistance referred to the meter resistance. 



An evaluation of the small error introduced by not providing th 

 damping resistor is as follows. Consider a square wave flux pattern pro 

 duced by an ideal electromagnet, with zero winding time constant o 



