measurements, developed either to make possible the measurement of 

 electrical quantities for which no methods of measurement existed 

 previously or else to make possible measurements in large numbers on 

 a routine basis at little expense which previously were delicate, 

 expensive and confined to the laboratories. It would be out of place 

 to include in this report a general discussion of the development of these 

 methods. Mention will be made, however, of certain items which 

 have particular reference to the new types of toll circuit discussed 

 above. 



An important tool in electrical measurements is the W'heatstone 

 bridge devised originally for the accurate measurement of resistances. 

 In dealing with telephone circuits where the performance of circuits 

 and apparatus in the transmission of alternating currents is important, 

 it was necessary to expand the Wheatstone bridge for alternating 

 current use. This involved providing elements for the Wheatstone 

 bridge having not only a known resistance to the flow of direct currents 

 but also a known resistance and reactance to the flow of alternating 

 currents of the frequencies at which measurements were to be made. 

 It was soon found, however, that with frequencies as high as those 

 required in telephone measurements, running up to two or three thou- 

 sand cycles, the resistance and reactance of the elements of the Wheat- 

 stone bridge were not well known and varied depending upon the 

 number of elements connected in the circuit. This variation was due 

 to the effect of incidental capacitances between the elements of the 

 bridge and between them and the ground. In order to overcome these 

 difficulties, G. A. Campbell devised an arrangement of shields by 

 which variation in the effect of these incidental capacitances was 

 prevented and in this way produced bridges for alternating current 

 use which would give accurate results over the range of frequencies 

 required in telephonic measurements. 



An interesting example of the application of the shielded impedance 

 bridge to practical telephone measurements is presented by what is 

 called the "capacity unbalance (testing) set." This testing set is de- 

 signed to measure the very small capacitances between individual 

 wires of a short section of toll cable or more specifically differences 

 between these capacitances for the individual wires of two pairs or of 

 two quads, expressing these differences in such a way that they are 

 directly proportional to the contribution made by the capacitances in 

 the short section of cable to crosstalk between circuits using the pairs 

 or quads thus tested. The purpose of the test is to give information 

 to the splicing forces, which, properly interpreted by them, enables 

 them to splice together pairs and quads in adjacent lengths in such 



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