=114. Ink Writing Registers= are frequently used instead of sounders. Fig. 114 shows a writing register that starts itself promptly at the opening of the circuit, and stops automatically as soon as the circuit returns to its normal condition. A strip of narrow paper is slowly pulled from the reel by the machine, a mark being made upon it every time the armature of an inclosed electromagnet is attracted. When the circuit is simply closed for an instant, a short line, representing a _dot_, is made. Registers are built both single pen and double pen. In the latter case, as the record of one wire is made with a fine pen, and the other with a coarse pen, they can always be identified. The record being blocked out upon white tape in solid black color, in a series of clean-cut dots and dashes, it can be read at a glance, and as it is indelible, it may be read years afterward. Registers are made for local circuits, for use in connection with relays, or for direct use on main lines, as is usually desirable in fire-alarm circuits. [Illustration: Fig. 114.] CHAPTER XV. THE ELECTRIC BELL AND SOME OF ITS USES. [Illustration: Fig. 115.] [Illustration: Fig. 116.] =115. Automatic Current Interrupters= are used on most common bells, as well as on induction coils, etc. (See § 104.) Fig. 115 shows a simple form of interrupter. The wire 1, from a cell D C, is joined to an iron strip I a short distance from its end. The other wire from D C passes to one end of the electromagnet coil H. The remaining end of H is placed in contact with I as shown, completing the circuit. As soon as the current passes, I is pulled down and away from the upper wire 2, breaking the circuit. I, being held by its left-hand end firmly in the hand, immediately springs back to its former position, closing the circuit again. This action is repeated, the rapidity of the vibrations depending somewhat upon the position of the wires on I. In regular instruments a platinum point is used where the circuit is broken; this stands the sparking when the armature vibrates. =116. Electric Bells= may be illustrated by referring to Fig. 116, which shows a circuit similar to that described in § 115, but which also contains a key K, in the circuit. This allows the circuit to be opened and closed at a distance from the vibrating armature. The circuit must not be broken at two places at the same time, so wires should touch at the end of I before pressing K. Upon pressing K the armature I will vibrate rapidly. By placing a small bell near the end of the vibrating armature, so that it will be struck by I at each vibration, we should have a simple electric bell. This form of electric bell is called a _trembling_ bell, on account of its vibrating armature. [Illustration: Fig. 117.] [Illustration: Fig. 118.]