combinations that the crosstalk unbalances in adjacent sections tend 

 to neutralize each other and the crosstalk between all pairs and quads 

 i n the cable when completed will be small. The capacitance differences 

 measured in individual lengths are only a few millionths of a micro- 

 farad. This measurement, originally possible only under carefully 

 controlled conditions, is, by the use of the capacity unbalance testing 

 set, reduced to a routine part of the work of the construction and cable 

 splicing forces. 



Another interesting kind of measurement bearing in a very im- 

 portant way on the maintenance of the efficiency of telephone toll 

 circuits is measurements of transmission efficiency, that is, of the 

 power output of the telephone circuit in proportion to the power input 

 of alternating current at the distant end. In order that such a meas- 

 urement may represent the efficiency of the circuit for the transmission 

 of telephone currents, it is necessary not only that the frequency of 

 the testing current correspond to one of the important frequencies of 

 telephone currents ^1,000 cycles is ordinarily used when only one fre- 

 quency is necessary) but also that the amount of power transmitted 

 correspond approximately to the average power of telephone currents. 

 For this reason the standard input power for such tests is one milliwatt 

 and the power received at the other end of the telephone circuit is often 

 one-tenth of that or less. 



When tests of this sort were first made as a part of the routine work 

 of maintaining telephone toll circuits the only available instrument 

 sufficiently sensitive to measure the received power and practical for 

 use under field conditions was the combination of the telephone re- 

 ceiver and the ear. In making such a measurement power was 

 transmitted alternately over the circuit to be tested and over an 

 artificial circuit whose efficiency was known and adjustable, the 

 adjustment being made until the received sound was equally loud in 

 the two cases. Then with the further development of the art sensitive 

 receiving instruments became available which were substituted for the 

 telephone receiver and the ear, the adjustment then being made of the 

 artificial telphone line until the received power as indicated by the 

 sensitive meter was equal to that received over the circuit under test. 

 The perfection of instruments of sufficient sensitivity for this measure- 

 ment and yet sufficiently rugged to be practicable for use by the regular 

 telephone maintenance forces constituted a great advance in the devel- 

 opment of measuring systems for telephone transmission. The most 

 satisfactory instruments of this type made use of vacuum tubes to 

 provide the necessary sensitiveness. 



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