Fig. 31
The key used by telegraphers is represented in [Fig. 31]. It has connected with it a switch to keep the circuit closed when the key is not in operation. The Morse code of signals consists of dots and dashes, when printed, as follows:
| a | . - |
| b | - . . . |
| c | . . . |
| etc. |
Operators learn to read the message by the intervals between sounds. A dot consists of two taps of the sounder with a short interval between, and a dash consists of two taps with a longer interval between. One tap of the sounder is caused by its descending upon the metal stop below and another by its rising against the upper stop.
Telegraph sounders are operated on about a quarter of an ampere of current if from a battery circuit, or on about one tenth of an ampere from a dynamo circuit. The dynamo circuit is supplied with more volts of electric pressure, and hence its power is ample to cause the armature to strike the metal stops hard enough to be heard by the operator.
For example a battery circuit may supply to the sounder a current with these characteristics:
2 volts × .25 amperes = .5 watts,
while a dynamo circuit may give: