1921.] Caldwell and Marsden.—Inductive Interference. 293 
annoyance. In telegraph circuits, chattering of the relays is caused, the 
intelligibility of signals is impaired, and the speed and ease of transmission 
are reduced. The induced currents arise from two causes—(1) electro¬ 
static, which is generally responsible for most of the interference to tele¬ 
phone circuits, and (2) electro-magnetic, which is more important from the 
telegraphic point of view. 
The electrostatically induced current consists of a charging-current 
across the neutral capacity of the power circuit and communication circuit, 
and therefore roughly proportional to the induced electrostatic voltage as 
described in the previous section. This current, being a capacity current, 
will have a magnitude proportional to the frequency of the transmission 
system. It will not be nearly so serious for an isolated metallic telephone 
circuit as for an earthed 
telegraphic circuit, because 
in the former case it will 
onlv be a differential effect 
a/ 
of the induced voltage on 
the two wires. In either 
case it will be considerably 
affected by the magni¬ 
tude of the self-induction 
associated with the com¬ 
munication circuit. 
In the case of a voltage 
wave (on the power circuit) 
which is not a pure sine 
wave, and which therefore 
contains higher harmonics, 
these higher harmonics will 
induce currents propor¬ 
tional to the product of 
their magnitudes and fre¬ 
quencies, so that relatively 
they will become more 
important. Moreover, not 
only are the higher har¬ 
monics relatively increased 
in the communication cir¬ 
cuit, but, up to frequencies 
of 1100, the intensity of sound produced by them is not the same for equal 
induced currents, but increases with frequency at a very rapid rate. 
For frequencies greater than 1100 the intensity of sound produced by 
equal currents falls off again with increased frequency. 
The sensitiveness of the average telephone-receiver to the same current 
at different frequencies has been determined by Osborne ( Proc . Am. I.E.E., 
vol. 38, p. 5, 1919). Multiplying each sensitiveness by frequency a curve 
(reproduced in fig. 6) is obtained, giving the relative amount of sound 
interference caused by unit voltage of different frequencies present in the 
harmonics of the power circuits. 
To obtain a factor giving the interference effect of several harmonics 
Osborne suggests that its value may be obtained by taking the square root 
of the sum of the squares of the separate interference effects. The factor 
so obtained is called the “ interference factor ” of a power circuit, and 
when sufficient practical data is accumulated this factor may be used as 
an interference wave factor for specifications. In the paper referred to a 
20—Science. 
frequenc y - Carre tor toferfsrence 
- IVare factor - 
Fig. 6. 
