308 
The N.Z. Journal of Science and Technolocy. [July 
Interference of Power Circuits with Telephone Circuits, by E. Parry. 
The question of the extent of the interference of power circuits with 
the working of Post and Telegraph circuits in their vicinity is an ever¬ 
present one to engineers engaged in these services. 
As the result of experience a considerable amount of information has 
been accumulated on the subject. Generally speaking, however, the 
present-day knowledge amounts to little more than a record of isolated 
events, and, so far as I know, no effort has been made to systematize the 
results in such a manner as to afford a means whereby results can be 
predicted under circumstances which differ from those prevailing at the 
time the observations were made. 
This paper is written partly with the object of establishing a general 
basis for co-ordinating the results of experience and for calculating before¬ 
hand the effects resulting from any given relative disposition of the circuits 
and from any given method of working, and partly with the object of 
determining approximately the intensity of the electromagnetic and electro¬ 
static fields surrounding the Lake Coleridge transmission-lines in particular, 
and is published for two reasons. One reason is that the Public Works , 
Department, in conjunction with the Post and Telegraph Department, 
is about to institute an experimental investigation into the influence of 
power circuits upon telegraph and telephone circuits, for which purpose 
a knowledge of the magnitude of the forces acting is desirable in order to 
design and adjust the apparatus to suit. The other reason is that telephone 
circuits for service purposes are supported on the same poles as the power 
wires on the system mentioned, and a knowledge of the magnitude of the 
indirect forces is necessary in order to appreciate the nature and extent 
of the protective apparatus. 
Dealing first with the effect of electrostatic induction : Referring to 
fig. 1, let A represent a conductor carrying a sinusoidal, electric current at 
a difference of potential between it and the earth of v a volts, and let D 
represent a telephone or telegraph wire subject to the influence of A. 
Let EE represent the ground, and A' and D' the images of A and D. 
Then if v A be the effective value of the voltage induced in D corresponding 
to v a in A, the following relationship may be proved to exist between 
the quantities involved :— 
v « = VaX HTTc a 
where M d is the mutual capacity of D and A with its image A', M a is mutual 
capacity of A and D with its image D', C d is the capacity of D to earth. 
Also— 
M d = 
where 
b 1 b 2 — apa. 
a , 
M =±__ 
XTJ -a 7 7 
a ± a 2 — o 1 o 2 
2 log e 
AA' 
a. 
2 log ( 
C d : 
DD' 
r& 
a . 
bA 
ct 1 a 2 
b 1=2 loge b 2 = 2 loge 
where r a is the radius of A and r d is the radius of D. 
Applying the foregoing to a three-phase transmission-system in relation 
to a telephone circuit: Let the three-phase conductors be arranged sym¬ 
metrically as in fig. 2, where they are represented by A, B, and C, and let 
D represent the telephone-wire. Dealing first with the electrostatic effect, 
* 0. Reavyside, Electrical Papers, vol. 1, p. 44. 
