In Fig. 69 you will note two groups of those instruments. Now observe how the wires connect them together. There are two line wires, one (A) which connects up the two batteries, the wire being attached so that one end connects with the positive terminal of the battery, and the other end with the negative terminal.Fig. 69. A Telegraph Circuit
The other line wire (B), between the two stations, has its opposite ends connected with the terminals of the electro-magnet C of the sounders. The other terminals of each electro-magnet are connected up with one terminal of each key by a wire (D), and to complete the circuit at each station, the other terminal of the key has a wire (E) to its own battery.
The illustration shows station 2 telegraphing to station 1. This is indicated by the fact that the switch F' of that instrument is open, and the switch F of station 1 closed. When, therefore, the key of station 2 is depressed, a complete circuit is formed which transmits the current through wire E' and battery, through line A, then through the battery of station 1, through wire E to the key, and from the key, through wire D, to the sounder, and finally from the sounder over line wire B back to the sounder of station 2, completing the circuit at the key through wire D'.
When the operator at station 2 closes the switch F', and the operator at station 1 opens the switch F, the reverse operation takes place. In both cases, however, the sounder is in at both ends of the line, and only the circuit through the key is cut out by the switch F, or F'.
The importance of the double click of the sounder will be understood when it is realized that the receiving operator must have some means of determining if the sounder has transmitted a dot or a dash. Whether he depresses the key for a dot or a dash, there must be one click when the key is pressed down on the screw head G (Fig. 62), and also another click, of a different kind, when the key is raised up so that its rear end strikes the screw head J. This action of the key is instantly duplicated by the bar D (Fig. 68) of the sounder, so that the sounder as well as the receiver knows the time between the first and the second click, and by that means he learns that a dot or a dash is made
To illustrate: Let us suppose, for convenience, that the downward movement of the lever in the key, and the bar in the sounder, make a sharp click, and the return of the lever and bar make a dull click. In this case the ear, after a little practice, can learn readily how to distinguish the number of downward impulses that have been given to the key.
Let us take an example in the word "electrical."
The operator first makes a dot, which means a sharp and a dull click close together; there is then a brief interval, then a lapse, after which there is a sharp click, followed, after a comparatively longer interval, with the dull click. Now a dash by itself may be an L, a T, or the figure 0, dependent upon its length. The short dash is T, and the longest dash the figure 0. The operator will soon learn whether it is either of these or the letter L, which is intermediate in length.
In time the sender as well as receiver will give a uniform length to the dash impulse, so that it may be readily distinguished. In the same way, we find that R, which is indicated by a dot, is followed, after a short interval, by two dots. This might readily be mistaken for the single dot for E and the two dots for I, were it not that the time element in R is not as long between the first and second dots, as it ordinarily is between the single dot of E when followed by the two dots of I.