1414 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



metrical magnetic structures having an infinitely long or torus shaped 

 round or rectangular cross-section, assuming linear magnetization char- 

 acteristics. However, for relay-like structures having air gaps, leakage 

 flux which only partly completes its circuit through the magnetic mate- 

 rial, varying cross-section so that boundary conditions become compli- 

 cated, and non-linear magnetic properties, an analytical approach be- 

 comes unmanageable. 



For a fundamental study and direct measurement of eddy current 

 effects, a test set has been developed to measure the dynamic flux rise and 

 decay characteristics of relays, and similar structures. This test set is 

 electronically operated on a synchronous switching principle. It displays 

 on an ordinary dc instrument, as a steady reading, the instantaneous flux 

 obtaining at any selected time after energization or de-energization of 

 the magnet. 



The application of the test set is restricted to devices which operate 

 under conditions of suddenly applied or removed dc voltage and which 

 can be cycled between these two conditions until the dc instrument 

 reaches its steady state reading. 



This article wiU present the theory of the basic synchronous switching 

 circuit and the relation applying between the dc instrument reading and 

 the instantaneous flux linkages, a description of the electronic control 

 circuits, and measurements of dynamic flux rise and decay using solid 

 core electromagnets. 



Supplementary additions to the basic circuit are available using the 

 same principles, to measure current-time, displacement-time, and ve- 

 locity-time curves of reciprocating devices. The first fundamental meas- 

 uring set using the switching principle, was built by E. L. Norton^ in 

 1938. The earlier set used a synchronous, motor driven, phase adjustable 

 commutator, to perform the switching. A limitation of the earlier set in 

 how fast it could be operated, combined with brush wear and chatter 

 troubles led to the development of the new electronically controlled set 

 employing sealed mercury contact relays to perform the switching. 



