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The rest of the apparatus consists of details and accessories for its action and adjustment, together with the sending-key K, which is used in asking for repetitions of the words, if necessary.
Wheatstone's next great invention was the automatic transmitter, in which the signals of the message are first punched out on a strip of paper, which is then passed through the sending-key, and controls the signal currents. By substituting a mechanism for the hand in sending the message, he was able to telegraph about 100 words a minute, or five times the ordinary rate.
Owing to induction, the speed of signalling on long cables is much slower than on land lines of the same length, and only reaches from 25 to 45 words a minute on the Atlantic cables, or 30 to 50 words with an automatic sending-key; but this rate is practically doubled by employing the Muirhead duplex system of sending two messages, one from each end, at the same time.
Obviously, if two kinds of current are used, that is to say, if the poles of the battery are reversed by the sending-key, and the direction of the current is consequently reversed in the circuit, there is no need to alter the length of the signal currents, because a momentary current sent in one direction will stand for a "dot" and in the other direction for a "dash."
Nestor was on his feet almost as soon as I. The three of us huddled about that little box as if our lives depended upon the message it had for us. Downes interrupted the clicking with his sending-key. The noise of the receiver stopped instantly. "Ask who it is, Downes," I directed. He did so, and while we awaited the Englishman's translation of the reply, I doubt if either Nestor or I breathed.
This neat little instrument is shown in figure 50, where M is the electromagnet, and A is the armature which chatters up and down between two metal stops, as the current is made and broken by the sending-key, and the operator listening to the sounds interprets the message letter by letter and word by word.
A current then flows into the line and traverses the receiver R' at the distant station, returning or seeming to return to the sending battery by way of the earth plate E' on the right and the intermediate ground. The duration of the current is at the will of the operator who works the sending-key, and it is plain that signals can be made by currents of various lengths.
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