The bell or alarum resembles in construction that of an ordinary clock, and is in fact a piece of clockwork wound up and ready to ring a bell, when the _detent_ or preventive is removed. The detent is connected with a piece of soft iron placed before an electro-magnet, and directly the current passes, the electro-magnet attracts the soft piece of iron attached to a perpendicular lever which the bell-crank lever rests upon; the detent is removed, and the bell rings, and again stops when the current of electricity ceases to pass. One of the most simple alarum clocks is a common American clock, wound up daily. A small electro-magnet surrounded with thick wire is placed below a moveable piece of tinned iron, so that when this is attracted, the fly of the clock is released, and its bell tolls unceasingly while [Page 223] the magnet is excited. This arrangement is employed by Sir W. O'Shaughnessy in the Indian telegraph system. (Fig. 212.) It will readily be comprehended from this description that the alarum is sounded by ordinary mechanism, and that the duty of the current of the electricity is simply comprised in the act of removing the lever and liberating machinery, which may be large or small; and if it were thought necessary, the bells of the great clock-tower of the Houses of Parliament, which chime the quarters, or even "Big Ben" himself (when his constitution is restored), could be rung by a person at York or Edinburgh, supposing wires, batteries, and a powerful electro-magnet with a detent mechanism for the bells, were properly arranged and connected with the clockwork. [Illustration: Fig. 212. A. The soft iron tinned, which is attracted to the electro-magnet B, and liberates the detent.] In certain cases, Mr. Charles V. Walker states that a single and distinct wire is used for the bell only, with his special mechanism, called the _ringing key_. If the bell was always on the same wire as the needle-coil, the bell would not only call the attention of, but seriously annoy the clerk (unless, of course, he happened to be a very deaf person) by its ringing whilst he was reading the signals of the needle. The nuisance is prevented by what is termed _joining over_ or making the _short circuit_--in fact, by providing for the current a shorter and much more capacious road to the needle coil than by going through that of the bell-magnet, which is made with very fine wire; and the control of the short circuit is put in the hands of the clerk. COOKE AND WHEATSTONE'S DOUBLE NEEDLE TELEGRAPH.