286 Scientific Proceedings, Royal Dublin Society. 
tor to the proper temperature, to dissipate the induced currents 
but the experiments were unsuccessful so far as I was able to 
carry them. 
The next attempts were made in the direction indicated by 
(c.), viz.—to eliminate the disturbing currents by their difference 
of period. Ifa medium existed which bore the same relation to 
electricity that, say, water does to radiant heat, we should be able 
to transmit through it waves of short period, but suppress waves 
of long period. But, as in electricity there is no phenomenon 
analogous to relative absorption in heat, the question must be 
attacked indirectly. This I endeavoured to do, in the first place, 
by splitting up the long waves into a series of waves of equal 
period ; these, if sufficiently rapid, would give rise to a musical 
note in the telephone which in its time might be got rid of by the 
principle of acoustic interference, A rapid current interruptor was 
made, consisting of a wheel driven by clock-work, and furnished 
with a break, so that various speeds could be obtained. Cuts 
were made in the periphery of the wheel, at equal intervals, and 
the cuts filled with an insulating substance ; against the edge of 
the wheel a spring pressed, through which the current from the 
line that entered the clock-work could pass to the telephone ; thus, 
after a momentary interruption, the current pressed for a longer 
period, followed by another momentary interruption. In this way 
from 10 to some 300 interruptions, per second, could be made, and 
it was hoped that, as the entire duration of contact far exceeded the 
duration of the interruption, the perception of the words spoken 
in the telephone would not be seriously interfered with, whilst 
the induced currents would be broken up into a series of currents 
of equal period. This was the case, and upon trying this arrange- 
ment, a rough musical note was heard in the telephone, the pitch 
depending, of course, on the current interruption. I hoped to 
suppress this note by using an interference apparatus I devised 
some time ago.* It consists merely of two brass tubes (Fig. 9), 
bent into a circular form, one sliding over the other; into each a 
straight mouth-piece opens; by turning one of the tubes round, 
the two mouth-pieces could be adjusted to any given distance 
from each other. A sound-wave entering the mouthpiece A can 
thus be made to travel over unequal distances in the two seg- 
* Philosophical Magazine, August, 1874. 
