WIRELESS TELEGRAPHY. 
115 
structed, and one, moreover, that is better able to respond to impulses 
which rapidly alternate in their character. This we find in the tele¬ 
phone, which takes the place accordingly of the galvanometer. In 
the transmitting apparatus we require to produce, (automatically for 
choice) sudden rises and falls of current (current rushes), and the more 
sudden the impulses the better will the distant telephone respond. In 
addition to this we want two parallel circuits of considerable length. 
I have here arranged two such parallel circuits a few yards apart 
across the room and completely separated from each other by an air 
space. One of the circuits (the receiver) is closed through a simple 
telephone. Tho other, the transmitting circuit, contains a battery, 
key, and, in this instance, a simple automatic interrupter. The in¬ 
terrupter “ makes " and “ breaks '' the circuit with considerable rapid¬ 
ity—about 2G0 times a second—and on holding the telephone of the 
receiving circuit to the ear, the current vibrations are detected. If 
the transmitting key be depressed for long and short intervals of 
time in accordance with the Morse code, the signals can be read ac¬ 
cordingly. 
(Experiment shewn :—Sending a message across the room, one of 
the audience taking the message on the telephone). 
The current interrupter used in this experiment is really a small 
motor used for driving a small ventilating fan. The fan is removed 
and an automatic interrupter substituted. The experiment just 
shown was carried out with what is really a model of Preece's system 
(applause). 
It appeared to me at first somewhat a disadvantage that the 
vibrations of that receiving telephone cannot be heard all over the 
room, but this fact will serve to emphasize the main drawback of the 
system, namely, the great weakness of these transmitted signals, 
natural enough, seeing that the energy which operates the telephone 
is ouly a very small fraction of the energy radiated and the source of 
the energy that produces the signals is situated at this distant station. 
I may at once attract your attention to the difference in the 
nature of the signals received with Preece's and with Marconi's ap¬ 
paratus. 
(Experiment shewn:—A few signals from a “ Marconi" trans¬ 
mitting apparatus sent to a “ Marconi" receiving set at the far end of 
the theatre. The noise of the vibrating hammer of the receiving set 
was heard all over the theatre). 
The apparatus which signalled is one I had specially made to make 
noise enough to be heard all over a large room ; it is not one of Mar¬ 
coni's own sets, though constructed on his principle. Marconi's sets 
are very much more delicate and sensitive. I shall shortly show you 
one (applause). In Marconi’s system a contrivance at the receiving 
station—which is very sensitive to these etherial waves—sets in action 
energy located at the receiving station. The audible signals are pro¬ 
duced, not by this very feeble amount of energy transmitted, but bv a 
local source of energy, viz : a voltaic battery put in action by the 
transmitted energy. There are many devices in common use for put- 
