SOME ASPECTS OF NOISE INDUCTION 481 



and exactly 120 degrees apart in phase. As a general rule, however, 

 the actual inductions from the different sections are not identical in 

 magnitude nor exactly 120 degrees apart in phase because of irregu- 

 larities in the pole spacing and dimensions of the parallel and because of 

 the fact that electrical waves take finite times to be propagated over 

 the wires and hence do not have the same phase in successive lengths. 

 For the usual distances encountered, the phase shift at fundamental 

 frequency is small but it may be appreciable for the higher harmonic 

 frequencies. 



The analysis outlined above for balanced voltages can also be em- 

 ployed for balanced currents. When the load on the power line is not 

 symmetrical the balanced currents will not be equal in magnitude and 

 exactly 120 degrees apart in phase even though the vector sum is zero. 

 However, these line currents may readily be divided into two sets of 

 currents each of which may be represented by a set of vectors of equal 

 magnitude and 120 degree phase displacement. The induction from 

 each set of vectors may be neutralized by powder transpositions (subject 

 to the same limitations as for balanced voltages) and it follows, there- 

 fore, that the induction will be neutralized for their combination. 



Transpositions in power systems affect the induction from residuals 

 only to such extent as they may affect the magnitude of the residual 

 voltages and currents (by providing better balance to earth). This is 

 because the residuals act on the wires in parallel (as pointed out 

 previously) so that interchanging the positions of the wires will not 

 directly affect the inductive field about them. 



To demonstrate the effect of power circuit transpositions on induc- 

 tion due to balanced and residual voltages, the miniature power circuit 

 can be transposed to form a complete barrel. When the power circuit 

 is energized with balanced voltages, a substantial reduction in noise 

 from the loud speaker occurs when the transpositions are cut in. When 

 the line is energized with residual voltage, however, cutting in power 

 circuit transpositions does not cause any change in the noise from the 

 loud speaker. In actual exposures, both balanced and residual vol- 

 tages and currents may be present so that the effectiveness of power 

 circuit transpositions will depend upon the particular conditions in each 

 specific case. 



Transpositions in Telephone Circuits 



As in the case of power circuits, telephone transpositions have, from 

 the standpoint of noise, two functions. The first is the equalization of 

 admittance unbalances to earth and to other conductors, of the conduc- 

 tors of the particular circuit under consideration. The second is the 

 reduction of noise due to direct metallic-circuit induction. CA third 



