292 The N.Z. Journal of Soience and Technology. [Jan. 
Furthermore, the quantity of electricity Q induced on the wires should 
they be earthed is roughly proportional to the voltage indicated above, 
so that the energy of discharge, which is proportional to QY, will fall off 
with distance at a rate proportional to the squares of the above figures. 
In actual practice it will seldom occur that a telephone-wire will run 
for the whole of its length parallel to a transmission-line, so that the effective 
capacity per unit-length concerned in the induction will be increased by 
an amount depending on the length of wire more remote from the 
transmission-line. This will reduce the induced voltage proportionally. 
In any particular case where the induced voltage should still prove 
excessive it may be reduced either (1) by adding capacity, (2) by trans¬ 
posing the transmission-line, (3) by drainage-coils. 
We have so far tacitly assumed that the voltage of the transmission¬ 
lines are the theoretical R.M.S. voltage to earth—namely, 66,000 -f- a/ 3 
— 38,100 volts. As the Lake Coleridge transmission system has delta-delta 
connections at the power-station and delta-star with grounded neutral at 
the substation, this will be exactly the case only if the capacities to earth 
of the three lines are equal. Since there are no transpositions in the 
system, this will not obtain, and consequently there will be a capacity 
unbalance with a consequent residual voltage : i.e., the sum of the voltages 
of the lines at any instant, instead of being zero as for an exactly balanced 
system., will give a residual potential. This residual voltage will have a 
much larger effect relative to its magnitude than the effects of the voltages 
in the individual lines, since by their phase relationship these will tend to 
annul each other’s effects. In this connection it may be noted that for 
lines separated by the width of an ordinary highway the vertical type of 
power circuit will, with balanced voltages, produce less interference than 
the triangular type, owing to the lines being more nearly equidistant from 
the communication circuit. 
In the tests described the effect of the residual voltage would be to 
increase the values of the induced voltages. On the other hand, the 
capacities of the wires to earth will be increased by the proximity of earth 
wires on the poles, &c., and this will tend to reduce the induced voltage. 
Such relatively small disagreement as is shown by the tables between 
calculated and observed induced voltages can probably be traced to these 
two effects. 
Transposition of a power line will have the following effects in the 
induced voltage in a neighbouring circuit:—- 
(1.) It will equalize the capacities to earth of the three lines, and thus 
minimize residual voltage. 
(2.) It will, by presenting different phases successively towards the 
neighbouring circuit, tend to annul the effects of the ordinary 
balanced components. 
The induction effect of a power system may be still further reduced 
by suitable transformer connections, such as delta-star at power-house 
with delta-star at substation, and with earthed neutral in each case or one 
case only. In the case of two transmission-lines running in parallel along¬ 
side one another the residual voltage, and consequently the induction effect, 
may be reduced without transposing by suitable arrangement of correspond¬ 
ing phases in the two lines. 
Part II.— -Current Interference. 
The current-disturbances in telephone circuits manifest themselves as 
humming noises which impair the intelligibility of conversation and cause 
