LEAD-COVKRED PAPER-INSULATED TELEPHONE CABLE 465 



bridge.'" Due to the fact that these bridges are frequently employed 

 in shop areas where some noise exists it has been necessary to develop a 



GALVANOMETER 



AYRTON SHUNT 



rM- 



^SHIELD 



ALL OTHER 

 CONDUCTORS' 



CONDUCTOR 

 UNDER TEST 



<0.l MEGOHM ^==" 



> PROTECTIVE 

 <" RESISTANCE 



-=- 500 

 ^ VOLTS 

 -[-DC ^ 



Fig. io — Shielded insulation resistance test circuit. 



device to replace the telephone receiver as a means of indicating 

 bridge balances. The visual bridge balance indicator used consists 

 essentially of a vacuum tube circuit in which the alternating current 



— NOTES — 



1 - SET RESISTANCE OF (A) EQUAL TO LENGTH OF CABLE 



2 - BALANCE OUT LEAD RESISTANCE WITH (B) 

 3- READ OHMS PER MILE AT eS^F FROM (C) 



SWITCH FOR MAKING 

 PRELIMINARY BALANCE 



Fig. 34 — Conductor resistance measuring circuit. 



" 5e// Sys. Tech. Jour., July, 1922: "Measurement of Direct Capacities," G. A. 

 Campbell. Transactions A. I. E. E., \'ol. XL\'l, May, 1927: "High Frequency 

 Measurement of Communication Apparatus," W. J. Shackelton and J. G. Ferguson. 



