For most purposes however another form called the trembler bell is much more convenient. Fig. 24 is a rough diagram of the usual arrangement of the essential parts of a trembler bell. When the circuit is closed by pressing the bell-push, a current flows from the battery to the electro-magnet EE, by way of terminal T. The electro-magnet then attracts the soft iron armature A, thus causing the hammer H to strike the gong. But immediately the armature is pulled away from the terminal T¹ the circuit is broken and the magnet loses its attraction for the armature, which is moved back again into contact with T¹ by the spring S. The circuit is thus again closed, the armature is again attracted, and the hammer strikes the gong a second time. This process goes on over and over again at a great speed as long as the bell-push is kept pressed down, resulting in an extremely rapid succession of strokes upon the gong. It will be noticed that the working of this bell is very similar to that of the automatic contact-breaker used for induction coils (Chapter VIII.). For household purposes this form of bell has completely driven out the once popular wire-pulled bell. Bell-pushes are made in a number of shapes and forms, and Fig. 25 will make clear the working principle of the familiar form which greets us from almost every doorway with the invitation, “Press.” In private offices and elsewhere the rather aggressive sound of an ordinary trembler bell is apt to become a nuisance, and in such cases a modified form which gives a quiet buzzing sound is often employed. It is frequently necessary to have an electric bell which, when once started, will continue ringing until it is stopped. Such bells are used for fire and burglar alarms and for many other similar purposes, and they are called continuous-ringing bells as distinguished from the ordinary trembler bells. In one common form of continuous-ringing bell two separate batteries are used, one to start the bell and the other to keep it ringing. When a momentary current from the first battery is sent over the bell lines the armature is attracted by the electro-magnet, and its movement allows a lever to fall upon a metal contact piece. This closes the circuit of the second battery, which keeps the bell ringing until the lever is replaced by pulling a cord or pressing a knob. Continuous-ringing bells are often fitted to alarm clocks. The alarm is set in the usual way, and at the appointed hour the bell begins to ring, and goes on ringing until its owner, able to stand the noise no longer, gets out of bed to stop it.