386 BELL SVSTEM lECHSUCAL JOURNAL 



but an\- 1)1 tlicni tciul to produce conditions which may be very 

 detrimental to service. These diagrams ilkistrate therefore, how 

 important it is to maintain the telephone plant so that troubles of this 

 nature will not be i)resent. 



(2) Measurements of 'rmitsiuissioii Gains. The transmission gains 

 of ami)lilier circuits .ire measured in inuch the same way as trans- 

 mission losses. A gain ma\- be considered as a negative loss and is 

 expressed in the .same transmission units. In measuring the gains 

 of amplifier circuits designed for two \\a\ operation, it is necessar\- 

 to pro\ide the proper balancing conditions in order to prevent "sing- 

 ing." This is done by connecting the amplifier circuit between two 

 artificial lines of the proper impedances and balancing these lines b\' 

 networks. The simplest measuring circuit now in use consists of an 

 arrangement whereby the repeater or amplifier under test is con- 

 nected between two artificial lines with .balancing networks and tone 

 is supplied by an oscillator at the ui ininal of one line while the termi- 

 nal of the other line is equipijcd with a reccixer and a measuring shunt 

 calibrated in transmission iiiiit>. The reixater and the shimt are 

 then alternateh' cut in and oiii oT tiic circuii and the shunt adjusti'd 

 until equal \oliimr of tour i^ oIimtxciI in tin- recei\er. for which 

 condition the sluml reading i;i\es llie gain ol tile repeater. 



A visual nu-tliod for nieasuriiig repeater gains is proxided b\- sub- 

 stituting an amplifier detector circuii for the shunt and receiver. 

 This is essentially what is done in the transntission measuring circuit 

 shown in Fig. 2."). The type of set designed for permanent installa- 

 tion which enii)lo\s this circuit is arranged so that amplifier gains 

 up to about 20 Tl' can be measured when a repeater is connected 

 in place of the lines under test and the necessary repeater balancing 

 reciuirements taken care of. The gains of repeaters connected in 

 toll circuits are often checked in this \\a>- wiieii owrall transmission 

 tests are made on these circuits. 



To meet practical testing reciuirements at the larger repeater and 

 carrier stations where a considerable amount of gain testing work is 

 done, a visual reading measuring set especially designed for testing 

 am]5lifier gains has lieen de\eloped. The measuring circuit employed 

 in this gain testing setTias been described.' The equipment going 

 to make up these sets, that is, the measuring shunts, artificial lines, 

 amplifiers, meters, etc., is mounted in compact form on standard 

 jjanels which can lie installed at convenient locations near repeater 

 and carrier etiuipment. A panel mounted 1,000 cycle oscillator is also 



»A. B. Clark, Bell System Tccliiiiait Jounuit, \oliimo II. .No. I, Jaiui.ity, 1923. 



