File:EB1911 Telegraph - Post Office Standard Relay.jpg

English: The most modern pattern of relay used by the British Post Office, known as the “Post Office Standard Relay.” In this instrument there are two soft iron tongues, n, s, fixed upon and at right angles to an axle a, which works on pivots at its ends. These tongues are magnetized by the inducing action of a strong horse-shoe permanent magnet, S N, which is made in a curved shape for the sake of compactness. The tongue plays between the poles of two straight electromagnets. The coils of the electromagnets are differentially wound with silk-covered wire, 4 mils (= .004 inch) in diameter, to a total resistance of 400 ohms. This differential winding enables the instrument to be used for “duplex” working, but the connexions of the wires to the terminal screws are such that the relay can be used for ordinary single working. Although the relay is a “polarized” one, so that it can be used for “double-current” working, it is equally suitable for “single-current” purposes, as the tongue can be given a bias over to the “spacing” side, i.e. to the side on which no current passes through the local circuit. The standard relay will work single current with a current of 3 milliamperes, though in practice about 10 would be used. Worked double current—that is, with the tongue set neutral, having no bias either to the spacing or marking side—the relay will give good signals with 1½ milliampere of current, though in practice 10 milliamperes are provided. The lightness of the moving part enables great rapidity of action to be obtained, which for fast speed working is very essential. The relay tongue, being perfectly free to move, can be actuated by a comparatively weak current. Normally a switch attached to the key cuts the battery off, and connects the line direct through the receiving relay; this switch is turned to “send” when transmission commences, and is moved back to “receive” when it ceases: this movement is done quite mechanically by the telegraphist, and as it is practically never forgotten, automatic devices (which have often been suggested) to effect the turning are wholly unnecessary.