INSULATION TEST EQUIPMENT FOR LOCAL CROSSBAR SYSTEMS 641 



desirable characteristics, is used as the current measuring instrument. 

 It has a low impedance input circuit, in which the low amplitude dc 

 leakage current flows. An alternating current of constant amplitude and 

 frequency is supplied to separate windings of the magnettor, so that, as 

 explained below, its output circuit delivers an alternating voltage which 

 varies with the dc input. Fig. 10 shows the basic circuit, which operates 

 as follows. Two identical mndings, a and a', and two other identical 

 windings, b and b', are wound on identical permalloy cores. Windings 



LINE LINK FRAME 



BAND 



TEST NUMBER 

 OFFICE 



u 



0-82 

 0-82 

 0-82 

 0-82 

 0-82 

 0-80 

 0-82 

 0-82 

 0-80 

 0-82 



-VERTICAL GROUP 



-HORIZONTAL GROUP 

 -VERTICAL FILE 



0801-01 

 0704-10 

 0705-22 

 0007-21 

 0205-32 

 •0406-63 

 •0800-74 

 ■0601-70 

 •0806-73 

 ■0100- S3 



Fig. 9 — Teletype record of failures. 



a and a' are connected in series and supplied with an alternating current. 

 The magnettor cores have a characteristic as sho^vn in Fig. 11 (b), and 

 the amplitude of the input voltage is great enough so that the core is 

 driven to saturation on each half cycle. Fig. 11 (c) shows the resulting 

 flat topped flux versus time curve*. Since the voltage induced in winding 

 b is proportional to the rate of change of flux, it will have a wave form 

 as sho^vn in Fig. 11 (d). The voltage peaks occur when the flux rate of 

 change is maximum, and during the "flat" intervals the induced voltage 

 is small. Since the b and b' windings are connected so that their output 

 voltages are in opposition (see Fig. 11 (d)) the net output with no dc 



* The wave shapes of Figs. 11 (c), 11 (d), 11 (e) and 11 (f) have been exagger- 

 ated to illustrate the action involved. 



