286 
The N.Z. Journal of Science and Technology. 
[Jan. 
INDUCTIVE INTERFERENCE OF POWER CIRCUITS 
WITH COMMUNICATION CIRCUITS. 
By A. C. Caldwell, B.E., and Professor E. Marsden, M.C., D.Sc. 
Part I.—Electrostatic Interference. 
The question of interference of power circuits with telephone circuits is 
becoming more and more important with the probable increase in the 
number of high-tension transmission-lines throughout the Dominion, especi¬ 
ally as many of these will have to be run through 
country containing a network of telephone and 
telegraph lines. 
Calculations of the effects to be expected in certain 
simple cases were made by Parry and published in this 
Journal (vol. 2, p. 308, 1919). Since then the following 
measurements have been made under the auspices of 
the Public Works Electrical Department to test the 
results of these calculations, and although these ex¬ 
periments are not yet finally concluded it appears 
desirable, because of the pressing importance of the 
question in New Zealand, to publish the results that 
have already been obtained. 
It will perhaps be advisable to recapitulate briefly 
Parry’s calculations. 
In Fig. 1, A is the high-tension line ; A' its image in the ground EE' ' 
D is the line whose induced voltage it is desired to calculate ; D' is the 
image of D. 
Parry states that— 
M d _ 
'd = '.x = «A (say). 
M a + c d 
where 
and 
M d = 
6 i 
1) ^ * 
, M.= 
a 1 a 2 — b J), 
o, 
a- 
t) ^ 
0 i M' 0 , DD 7 
«i = 2 lo ge-> a 2 = 2 log e _, 
La r d 
= 2 lo g e h 2 = 2 log e — • and 
DA 
AD ’ 
where r a , r d are the radii, and v A , v A the potentials of A and D respectively. 
By introducing two more similar terms for the other conductors B and C 
of a three-phase system we obtain for the total potential of D 
v i = V A + + V A- 
In th e case of the Lake Coleridge transmission-line (66,000 volts between 
phases) the R.M.S. values assumed throughout are 
v A = 38100, 
«b = 38100 (- i + j v 2 3 ), »c = 38100 (- i - j 
the vector v A being chosen as the vector of reference. 
