SOME ASPECTS OF NOISE INDUCTION 493 



The grounded circuit represents completely unbalanced conditions 

 since the sides of such a circuit have a separation comparatively great 

 compared to that of a metallic circuit. Consequently, the inductive 

 susceptiveness of a grounded circuit is much greater than that of a 

 metallic circuit, even if the latter is not transposed. Furthermore, 

 a grounded circuit cannot be transposed practicably. To illustrate the 

 difference in the susceptiveness of the two types of circuits, the tele- 

 phone circuit of the demonstration set up has been arranged as shown 

 schematically in Fig. 17 so that either of the two types of circuits may 

 be obtained. The power circuit arrangements are as shown in Fig. A-B. 

 The large reduction in the noise from the loud speaker which occurs 

 when the connections are changed from grounded to metallic, shows the 

 decreased susceptiveness of the latter type of circuit. 



GROUNDED 



r 



TO AMPLIFIER 



TELEPHONE LINE J -J- AND 



IN EXPOSURE J ^ ^ - LOUDSPEAKER 



Fig. 17 — Comparison of noise in metallic and grounded circuit. 



For metallic circuits, the inductive susceptiveness depends on a 

 number of factors such, for example, as the spacing of the wires, the 

 power levels, and the circuit balance. Some of these are discussed 

 below. 



Spacing 

 Since the direct metallic induction (which, as discussed before, is a 

 function of the difference of the voltages induced on or along the two 

 sides of the circuit) is about proportional to the distance between the 

 two sides of the circuit, this separation is of interest from the stand- 

 point of the circuit susceptiveness. The smaller the spacing of the 

 wires, all other things remaining the same, the smaller ordinarily will 

 be the direct metallic induction and the noise-metallic from this source. 



Power Level 

 Another important element in determining the inductive susceptive- 

 ness of a telephone circuit is the power level of the telephone waves 

 transmitted over the circuit. The more powerful the telephonic cur- 

 rents at a point, the less they will be interfered with by a given amount 

 of noise power which may be induced in the circuit at that point. This 

 is particularly important on long toll circuits where the telephonic 

 power level may be materially affected by the spacing, power carrying 



