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BELL SYSTEM TECHNICAL JOURNAL 



only one amplifying vacuum tube, but additional amplifiers may be 

 added to meet the requirements of particular kinds of testing work. 

 One of the latest forms of oscillators of this type is shown in Fig. 27 

 set up for use with one of the permanent types of transmission meas- 

 uring sets. 



Vacuum lube ()S(ilicit(H> lia\e been tle\el(iiK-<l which will generate 

 measuring currents of any desired frequency within the range of 100 

 cycles to 50,000 cycles, thus covering both the voice and carrier 



Fig. 31 



range. These oscillators liaxe become in<lis]XMisabk' in ti'siing and 

 maintenance work. They are used extensi\ely in making both single 

 frequency transmission tests and transmission loss and gain tests 

 within the range of frequencies mentioned above. They are also 

 used in making line impedance and impedance unbalance tests and 

 in determining the characteristics of telephone repeater and carrier 

 circuits. 



Specific Applications of Electrical Testing Methods. In describing 

 the various electrical tests above, considerable has been said regarding 

 the applications which are made of them to insure satisfactory tele- 

 ]>hone transmission. In order to give an o\erall picture of these 

 ajiplications the toll connection for which transmission level diagrams 

 are given in Fig. 28, is shown in simplified form in Fig. 32, with \arious 

 tests listed underneath the different sections of the circuit la>out. 

 Only the sections of the circuit making up the first part of the con- 

 necti(jn are shown since corresponding tests will ajiply to the circuits 

 making up the second. 



The tests listed in Fig. 32 are not intended to give a testing i)rogram 

 but rather to show the various electrical testing means which are 

 a\ailable for use in installation and maintenance work. Just what 

 tests should be made, the fre(iuonc>- of making the tests and the 

 limits to work to, to insure a high grade of transmission depend on 

 the types of circuits and equipment involved and their relative im- 



