In passing through the instrument the current proceeds from one terminal, say that on the right, by the wire W to the screw contact T, and thence by the spring S through the bobbins of the electromagnet to the other terminal. The electromagnet attracts the armature A, and the hammer H strikes the gong; but in the act the spring S is drawn from the contact T, and the circuit is broken. Consequently the electromagnet, no longer excited, lets the armature go, and the spring leaps back against the contact T, withdrawing the hammer from the gong. But the instrument is now as it was at first, the current again flows, and the hammer strikes the gong, only to fly back a second time. In this way, as long as the button is pressed by the operator, the hammer will continue to tap the bell and give a ringing sound. Press-buttons are of various patterns, and either affixed to the wall or inserted in the handle of an ordinary bell-pull, as shown in figure 86. The ordinary electric bell actuated by a battery is liable to get out of order owing to the battery spending its force, or to the contacts becoming dirty. Magnetoelectric bells have, therefore, been introduced of late years. With these no battery or interrupting contacts are required, since the bell-pull or press- button is made in the form of a small dynamo which generates the current when it is pulled or pushed. Figure 87 illustrates a form of this apparatus, where M P is the bell-pull and B the bell, these being connected by a double wire W, to convey the current. The bell-pull consists of a horseshoe magnet M, having a bobbin of insulated wire between its poles, and mounted on a spindle. When the key P is turned round by the hand, the bobbin moves in the magnetic field between the poles of the magnet, and the current thus generated circulates in the wires W, and passing through an electromagnet under the bell, attracts its armature, and strikes the hammer on the bell. Of course the bell may be placed at any distance from the generator. In other types the current is generated and the bell rung by the act of pulling, as in a common house-bell.