116 
WIRELESS TELEGRAPHY. 
ting in action energy situated at a distant station, and for an example 
I might give you the illustration of firing a gun electrically. You 
have a gun tube in the gun, a key some distance off, and a battery 
located anywhere; and the only amount of energy that you have to 
put into the circuit is the small amount required to depress the key, 
but that sets in action the battery ; the battery sets in action the gun 
tube, and the gun tube brings in action the huge amount of potential 
energy stored up in the cartridge. So that you see that there is 
nothing new in the arrangement of using a small amount of energy to 
bring into action a much larger amount. It is done in ordinary tele¬ 
graphy by means of instruments called “ relays.” 
Before passing on to the consideration in detail of Marconi's 
system I would like to show you a diagram on the screen of Preece’s 
system. It will be interesting to many of: you who are in the habit of 
following out electrical diagrams, to observe the method adopted by 
Mr. Preece to secure the desiderata above-mentioned—viz : great 
suddenness (see Plate I) in the making and breaking of as large a 
current as possible. [Diagram No. 1 was thrown on the screen.] 
The diagram (Plate I) shows that on both sides of the channel to 
be crossed, the ordinary <( Morse Key ” circuit is made. The current 
interruptions are produced by a commutator driven by a small 
auxiliary motor. The suddennes of the break is increased, and the 
sparking reduced by a condensor placed across the commutator ter¬ 
minals. I suppose at Oban the length of this wire was about 3 miles, 
and each end was connected to earth plates. Exactly similar circuits 
being arranged on both sides of the channel, a buzzing in the tele¬ 
phone at the distant station is produced as long as the Morse key 
at the near station is kept depressed. Hence signals can be exchanged 
between the two stations by the Morse code at about the same speed 
as ordinary telegraph signals. 
Mr. Preece says that for good effect it is necessary to erect 
parallel overhead wires of similar length to the distance to be crossed 
on each side of the channel. You will remember that I told you that 
in this system you really need more wire than is necessary for connect¬ 
ing directly. For good effect you require as much wire on the one 
side of the channel as you have on the other, and parallel to it. 
When earth returns are used in each circuit it is necessary to stretch 
the wires as much as possible above the surface of the ground. Some 
of the disadvantages of the system will at once be obvious. (1)— 
The extreme weakness of the signals renders it impracticable to call 
attention. It is impossible for an operator at a distant station to 
know that a message is being sent unless he has his ear perpetually at 
the telephone. (2)—The system is only possible when two parallel 
wires of considerable length can be stretched at some height on either 
side of the channel to be crossed. This renders it impossible on a 
lightship or rock lighthouse. There are many small islands for 
example where it cannot be done. On the other hand when signals 
are successfully transmitted they are very clear and very easily read, 
and the signals are not likely to be confused by disturbances from 
