ELECIKUJI. il-.SIS .l\l> nil Ik ;/777('. ///O.VV Mi.\ 



Ifrniinatiii^ tin- tnll riniiiis .md ilu' appar.iuis ,iss;K'iaU'<I willi iliciu. 

 such as phantom aiul siini)U-\ coils, composite sets. etc. The 100. (KM)-, 

 ohm \oltmeter ami the W'healstoiie bridge ami keys for ol>taiiiii)){ 

 various testing arranj-emeiits are mounted in the horizontal shelf and 

 connections are made to the toll circuit and ecjuipment jacks 1)\- 

 means of the cords and plu^s located at the back o( the shelf. The 

 tek'vjraph instruments are used on order wires lo distant test boards 

 and the meter shown in the vertical section of the board is for measur- 

 ini; the voltage and current in telegraph circuits. 



The test board shown in Fig. 9 is designed primariK- for testing 

 the local plant, although tests on toll circuits can also be made from 

 this boartl. One transmission feature pro\ided in the board is an 

 artificial line which when cut in circuit with a 500 ohin subscriber's 

 Uxjp, gives an oxerall eciui\alent of approximately 30 TV. This 

 line is terminated on keys by means of which it can be connected 

 as a trunk circuit and used in talking tests on subscribers' loops at the 

 time of their installation or when subscribers' stations are N'isited in 

 connection with trouble complaints. Jacks are pro\ided in the 

 vertical section of the board for terminating certain test trunks and 

 other test trimks are terminated on keys. A Wheatstone bridge is 

 not normally mounted in this t\pe of test board, but where required, 

 a portable bridge is supplied which is generally kept in one of the 

 flrawers of the board when imt in use. 



.\i.ti:rnatin<, ("iRRi;\r 'I'i;sts 



While the direct current tests just described tell a great deal about 

 the physical and electrical condition of telephone circuits, it is %'ery 

 necessary in maintenance work to consider also the alternating current 

 characteristics. The transmission of speech is, of course, funda- 

 mentalK' a problem of the transmission of alternating currents of very 

 small \alues. The inductance and capacity as well as the resistance 

 and leakage of circuits, therefore, become important items in de- 

 termining the efficiency of telephone circuits and means must be pro- 

 vided for testing these characteristics under operating conditions. In 

 principle, alternating current testing mcthotls do not differ materially 

 from direct current methods and their applic.ition in the telephone 

 plant is not difticuh. 



Alternating Current Bridge Measurements. These measurements 

 employ Wheatstone bridge arrangements, the direct current source 

 of power being replacetl by an alternating current source and the 

 condition of bridge balance being obtained by some alternating 



