4 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1954 



tics — both for contact arrangements, and for magnets. Another field of 

 measurement involves magnetics. Both the mechanical output and the 

 electrical characteristics of the electromagnet are determined by its mag- 

 netization relations, requiring the measurement of magnetic flux. Beside 

 such static measurements of mechanical, electrical, and magnetic quan- 

 tities, corresponding measurements must also be obtained as functions 

 of time in studying relay behavior under the dynamic conditions of 

 actual operation. 



The many instruments which furnish such data may be grouped into 

 those for static measurements, and those for dynamic measurements. 

 Some of these tools are described in the following pages, particularly as 

 they relate to force, current, flux, displacement, time, or their combina- 

 tion. 



STATIC MEASUREMENTS 



The Measurement of Force 



For a complete understanding of the operation of rela}^ designs, knowl- 

 edge of how the forces vary as the magnet air-gap changes, or as the 

 contact members are deflected through their stroke, is of course required. 

 The measurements most needed concern the force versus distance of the 

 contact loads which the magnet must operate; and the force versus air- 

 gap characteristic of the associated magnet. For many years, such meas- 

 urements were made by a process of hanging weights and setting a 

 micrometer screw, point by point, until complete data were obtained. 

 More recently special instrumentation has made available a pendulum- 

 type tensile tester, and a spring balance device, which have greatlj^ 

 increased the convenience of making these measurements, point-by- 

 point. Today, however, many such measurements may be made still 

 more conveniently by a machine which automatically causes the gaps to 

 vary and plots a curve of force ^'ersus distance. Machines of this type 

 have been developed to a high degree of flexibility for tensile testing of 

 materials such as metals, plastics, textiles, or paper. 



One such machine, which is extremely convenient for relay measure- 

 ments, is the Instron tensile testing machine shown in Fig. 1. This ma- 

 chine is manufactured by the Instron Engineering Corporation of Con- 

 cord, Mass. In conjunction with suitable current supplies to control the 

 behavior of the relay magnet, and with a few special circuits for auto- 

 matically correcting for flexure of the relay parts, and making other 

 similar adjustments, this machine has proven eminently suitable for ob- 

 ser\'ing all "static" force-deflection characteristics of relays. The manner 



