36 
THE TELEGRAPH, 
expected. Further, if the clerk should have made a mistake 
and the signal be over, there is no further indication ; and it 
would be necessary to wait until it were renewed, as there 
would be no index to show which number had called. 
Evidently then, when subscriber No. 5 calls, there must be 
some visible and continuing sign made at the office, showing 
clearly that No. 5 is waiting. 
When a battery is used this is a simple matter, and when we 
speak of Edison’s telephone we shall explain how it is accom¬ 
plished ; but the Gower telephone has no battery, and if its 
simplicity is to be preserved it must dispense with it. If the 
use of the battery has some advantages, there are also some 
inconveniences. A battery increases the sound transmitted, 
and is said to simplify the signals. This may be true; but on 
the other hand the battery involves expense, and whatever 
may be its form, is necessarily consumed so long as the cur¬ 
rent passes. If the subscriber should forget to turn his 
commutator, the battery is exhausted in a night, and on the 
morrow the apparatus fails to speak. If all this can be 
avoided so much the better. But the difficulty in question 
is a serious one, and we have to thank M. Ader for solving it 
by the exhibition of a visible sign showing the correspondent’s 
call. 
The visible sign may be the shifting or fall of some piece of 
the apparatus, or a change of colour. In every instance it is a 
movement which must be effected by the telephone. * But the 
instrument yields only vibrations, and it is these which must 
be transformed into movements. Fig. 9 will show how this 
is accomplished : A is the magnet of a telephone, and the wire 
proceeding from the line is wound on the bobbins bb. The 
vibrating plate of the telephone is reduced to the little strip R 
fixed against s. The white disc bearing the word R&pondez 
(Answer) is the signal. In the position represented in the 
figure it is concealed; but its weight tends to make it fall, 
and when it has done so it becomes visible through an opening- 
made in the front of the case, which in the figure is removed 
in order to show the mechanism. 
As here shown it is prevented from falling by a lever L to 
which its upper part is attached, and which is provided with a 
suspended hook which catches in a square hole, o, made 
