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Proceedings of the Royal Society 
constructed so as to isolate this effect, and, if possible, bring it only 
into view. It consists of a short vertical wooden tube, containing a 
column of mercury, — the bore being about half an inch in diameter. 
On the top of the tube, which terminates in a circular enlarge- 
ment, an ordinary ferrotype plate is placed, which has a copper 
wire soldered to its centre, dipping vertically downwards for a 
short distance into the mercury in the tube. The current is led 
upwards through the mercury column to the copper wire, and thence 
to the ferrotype plate, which is in metallic connection with one of 
the poles of the battery. By this arrangement any action from self- 
induction which takes place will be of the nature of a thrust vertically 
upwards upon the copper wire, and through it upon the ferrotype 
plate, which will, in consequence, be made to vibrate in sympathy 
with the interruptions of the current in the circuit. When the 
tuning-fork was used, I was able to hear the sounds from it faintly, 
but still distinctly enough to leave no doubt of the effect. 
I next determined to try the effect of exhausting the air from 
the space surrounding the glowing spiral of fine wire. For this 
purpose I employed the apparatus known as the electric egg. A 
spiral of fine platinum wire was placed vertically between the two 
terminals, and I first made sure of hearing the sound of the fork 
by applying my ear to the outside of the egg. The air was then 
gradually exhausted, and the sound decreased perceptibly till it 
became almost inaudible. To get the effect of a more perfect 
exhaustion, I next employed one of Mr Swan’s electric lamps. When 
the interrupted current from the fork was sent through the lamp, 
no trace of sound could be heard by applying the ear to the side of 
it, although the variation of the glow in the carbon fibre from dull 
red to a very bright red, approaching to white, was most beautifully 
seen to be produced by the variation of the current. This at once 
shows that the presence of air or some other medium is necessary, in 
order that its expansion and contraction, produced by the heating 
and cooling of the wire, may affect the sides of the vessel, and so 
give out the sounds. 
