This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1884 edition. Excerpt: ...system a number of stations are placed upon one circuit, and when no message is passing along the line the cyrrent is circulating through all of them. On each key is a switch, H, which on being turned in one direction or the other breaks the circuit or closes it. The action of the electro-magnet at each station is, however, merely to close the local circuit, which operates another electro-magnet placed upon a resonant sounding base. This electro-magnet with its sounding base is termed a " sounder." Letting Fig. 90 represent a closed circuit, the current passing from the positive pole of the battery, through the key K, whose switch is closed, through the electro-magnet M, the line-wire, the electro-magnet M' at the receiving end and the key K', whose switch is closed, to the ground G'; the negative pole of the battery being connected to the ground at G. It will be noticed that only one wire is used. The reason for this is that for the small currents used in telegraphy the earth can act as a very good return conductor, the circuit being completed between the earthplate sunk in the earth at G' and one sunk in the earth at G. In this way we get a conductor for the return current costing nothing, and offering much less resistance than a wire. Now, the current, in passing through the electro-magnets M and M', magnetizes them, so that they attract their armatures, a and a'. This brings the armatures down upon contact-points, thereby closing the circuits of the local batteries, B' and B," magnetizing the electro-magnets M" and M'," which attract their armatures, and, being placed upon sounding bases, give out, therefore, very audible clicks. The current remaining closed, the armatures rest quietly in this position....