A still further improvement has been made by development of 

 instruments which within a limited range showed directly by their 

 amplitude of deflection the amount of power loss in the telephone 

 circuit. With this latter development the artificial circuit is entirely 

 dispensed with, the standard amount of alternating current power 

 applied to the circuit at one end and the meter connected to the other 

 end. These great advances in the technique of measuring instruments 

 provided an ease of measurement almost comparable to the ease of 

 the measurements commonly made in power transmission systems 

 where the large amounts of power available made the development of 

 satisfactory instruments very much less difficult. Now a still further 

 advance in these methods of measurement has been made by devising 

 arrangements such that the deflection of the instrument is indicated 

 in an enlarged scale on an illuminated screen. This makes it un- 

 necessary to transport the instrument to the terminal of the circuit 

 and makes it possible in a repeater office, by making connections in 

 one part of the room so that the circuit is connected to the receiving 

 instrument, for the maintenance man to read the deflection of the 

 meter at a distance thus further cutting down the time required for 

 tests of this nature. 



Other types of tests on telephone toll circuits for which special 

 measuring apparatus and measuring methods have been devised 

 include measurements of the crosstalk between circuits, measurements 

 of the noise currents induced in circuits by other electrical circuits, 

 such as electric power circuits, measurements of the uniformity of 

 electrical impedance from the standpoint of suitability for operation 

 with repeaters, measurements of the amplification of telephone re- 

 peaters and measurements of the thermionic activity of the vacuum 

 tubes. 



While the above discussion refers to instruments for the measure- 

 ment of alternating currents in what is called the voice-frequency 

 range, that is, up to about 3,000 cycles per second, the introduction of 

 carrier telephone and telegraph systems made necessary the develop- 

 ment of similar measuring instruments for the higher frequency cur- 

 rents used in carrier, namely, up to about 30,000 cycles per second. 

 With the expected use in the future of currents up to frequencies of 

 100,000 or 1,000,000 cycles or more the range of field measuring ap- 

 paratus will, of course, have to be greatly increased. 



The use of these special types of apparatus for making necessary 

 electrical measurements has required a large amount of instruction 

 of the maintenance forces. Also, it was necessary to devise systems 

 of test and adjustment using these measuring methods by means of 



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