68 On the Municipal Electric.Telegraph, 
groups numbers from one to seven cams, according to the num- 
r of the District in which the Signal box is placed. This com- 
municates the District number to the Central Office. The other 
group consists of from two to four cams, placed closely together, 
an as to complete the circuit for longer or shorter 
periods and produce a record at the Central Office, of dots and 
lines, indicating the number of the Station. The box contains 
instructions to turn the crank six times. The effect of this, with 
handle always carries the Crank back to its original position. 
The record made at the Central Office by the revolution of the 
Signal Crank can not be imitated by any person not having ac- 
cess to the Signal Boxes. A great additional safeguard from any 
abuse of the wires is thus obtained. 
The Signal Key © can be used in the ordinary manner, to 
communicate to the Central Office any system of Signals which 
may have been agreed upon for police or other purposes. Com- 
munications may be received, in return, from the Central Office, 
by means of the little electro-magnet and armature D, which is 
introduced in the course of one of the Signal wires. The click 
of the armature constitutes here the audible signal. This would 
also be heard on operating the Signal Crank or Key, if the cit 
cuit was duly completed, of which it would thus give indication. 
It would be heard also if another Station was in the act of com 
municating to the Centre, and it would thus prevent confusion in 
signalizing 
nen depressed by the cams, The electro-magnet | 
D would be included constantly in the eircuit, and the. click . 
