ELECTRICAL TESTS AND THEIR ATT LIGATIONS ,1K7 



pruvitlotl to sup()ly measiiriiin rurrcnt, although other types of oscil- 

 lators giving the necessary output and pro[x;r wave shapn; can be used . 

 if desiretl. 



In practice, it is necessary to maintain the gains of the ainpiiliers 

 in rejx-ater and carrier circuits to fairly close limits since these ampli- 

 fiers form an integral part of toll circuits. Measurements of gains 

 are also made in connection with the 21 circuit balance tests previ- 

 ously descril)etl. Another important application of gain tests is to 

 check the gain frequency characteristics of repeaters to determine 

 that all frequencies within the voice range are l)eing properly ampli- 

 lie<l. r^y varying the filament current between limits, a test of the 

 vacuum tul)es for filament activity is obtained by gain measurements. 



(3) Measurements of Transmitter and Receiver Efficiencies. Trans- 

 mitters and receivers are used in the telephone plant principally in 

 operators' sets and subscribers' sets. In the former, the transmitters, 

 receivers and operators' circuits are readily available to the main- 

 tenance forces and therefore can be inspected and tested in a routine 

 manner. In the case of subscribers' sets, however, the equipment 

 in service is not accessible and tests must be made on the instruments 

 liefore installation or at times when they arc removed from service. 

 Talking tests can also Ixj made from the instruments at the time 

 installations are made and an\- particularly unsatisfactory- conditions 

 found in this wa\-. 



The difficulties incident to testing transmitters and receivers arc 

 due to the fact that in transmitters, the efficiency depends on the 

 ability to convert sound energy into electrical energy and in receivers, 

 the ability to convert electrical energy- into sound energy-. Obviously, 

 a simple form of transmitter test and one which has until reccntly 

 IxHJn generally used is to talk alternately into the transmitter under 

 test and then into a standard transmitter and observe the difference 

 in volumes at a receiving set. In the same manner, a simple receiver 

 test is to listen alternately to a receiver under test and then to a 

 standard receiver connected to a talking station. This method is 

 slow and also of limited accuracy due to inherent changes in a speak- 

 er's voice and to the possibility of the distance of the speaker's lips 

 from the transmitter varying. To take the place of this method 

 transmitter and receiver testing machines have been developed which 

 will Ix- descrilied in a paper to Ix; published later. 



Oscillators. Practically all alternating current testing work recjuires 

 the provision of an external source of measuring current. For this 

 purpose oscillators of various types have been dev-elof)ed which are 

 designed electrically and mechanically to meet various test circuit 



