that it must rest against either the one or other pole of the electro-magnet below. If the north pole of the bar magnet rests against the right hand pole of the electro-magnet when the number does not show, we can cause the bar magnet to cross over to the other pole, and display the number by sending a current through the electro-magnet in such a direction as to make its right hand pole a north pole, and its left hand a south pole. This is because the two north poles will repel each other, while the south will attract the north. On being once tilted over, the bar magnet cannot return to its former position, until the person who used the bell sends a current in the opposite direction (which he can do by means of a reversing switch), when the poles of the electro-magnet being reversed, the bar magnet will be pulled back into its original position. Indicators of this class, owing to the fact that their replacement depends on the polarity of the bar magnet, are also known as "polarised indicators."
Fig. 69.
Fig. 70.
§ 65. For general efficiency and trustworthiness, the pendulum indicator; as shown at [Fig. 70], is unsurpassed. It consists of an electro-magnet with prolongation
at the free end on which is delicately pivoted a soft iron armature. From the centre of this armature hangs, pendulum fashion, a light brass rod carrying a vane of fluted silver glass, or a card with a number on it, as may be found most convenient. This vane or card hangs just before the aperture in the indicator frame. Stops are usually placed on each side of the pendulum rod to limit the swing. When the electro-magnet is magnetised by the passage of the current, the armature is pulled suddenly on one side, and then the pendulum swings backwards and forwards in front of the aperture for some minutes before it comes to rest. When fitted with silver fluted glass, the motion of the vane is clearly visible even in badly lighted places. As the pendulum, after performing several oscillations, comes to rest by itself in front of the aperture, this indicator requires no setting. Messrs. Binswanger fit these indicators with double core magnets, and have a patented adjustment for regulating the duration of the swings of the pendulum, which may be made to swing for two or three minutes when the circuit is completed by pressing the push; it then returns to its normal position, thus saving the servant the trouble of replacing the "drop."