114 
WIRELESS TELEGRAPHY. 
Let me show you the fundamental experiment on which the 
system depends. I am rather ashamed of the simplicity of the 
contrivance, and the only excuse I have for showing it to you is that 
this is the original experiment of Faraday and carried out probably by 
the very apparatus that Faraday used. Here it is (showing it). It 
was picked up covered with the dust of years at the K.M.A., where 
it had been lying, I am afraid to say how long; and of course you 
know that Faraday was once science lecturer at that Institution. 
First of all a little explanation is necessary before showing the 
experiment itself. One of the best known phenomena connected 
with the electric current is, that in the vicinity of a wire conveying 
it, magnetic forces are experienced which disappear when the current 
ceases to flow. Now I will ask you to imagine a wire stretching 
from this wall to that across the room. Suppose a current to be 
started in that wire by depressing a key. The current at first is 
zero in strength and rises to its steady maximum value, taking 
usually only a very small fraction of a second to do so. During the 
time of the current rising there are thrown off from the wire con¬ 
veying it, as from a centre, a so-called electro-magnetic wave. 
Suppose another parallel wire stretched across the room and form¬ 
ing part of a closed circuit, the expanding electro-magnetic wave 
will wash this second circuit, or a portion of it, and a converse action 
will take place, that is to say a momentary current in this second cir¬ 
cuit will be induced. I say momentary, because this wave motion 
ceases when the current has attained its steady value. Suppose the 
current in the first wire to be now suddenly broken; the ether is re¬ 
leased from a state of strain, and a wave in the reverse direction re¬ 
sults, causing a second momentary current in the distant circuit. 
(Experiment shewn :—Two parallel coils of insulated wire, separ¬ 
ated from each other by a few inches of air. One coil connected with 
a battery and key—the circuit of the other closed through a galvan¬ 
ometer. On depressing key, a momentary disturbance on galvano¬ 
meter results. On releasing key another momentary swing of galvan¬ 
ometer needle in opposite direction to the first is seen). 
In the experiment just shown, when the key is depressed I 
ask you to imagine an electro-magnetic wave, which starting from 
the surface of the first coil, washes the second coil whose circuit is 
closed through the galvanometer, and you notice the galvanometer 
needle swing. On releasing the key the strain is released, and the 
return wave is started. Accordingly a momentary deflection on gal¬ 
vanometer, but in the opposite direction results. The effects are small 
and weak, you see, in the extreme, but that illustrates from what 
small beginnings practical results sometimes follow (applause). 
This, then, is the laboratory experiment, the parent of the system 
of the wireless telegraphy we are considering. We can gather from this 
experiment what are the desiderata to enable the actions just described 
to take place across sufficient space for practical use in transmitting 
signals. In the receiving apparatus, first of all, we must have an in¬ 
strument very much more sensitive than any galvanometer yet con- 
