=173. Call Boxes= are used to send in calls of various kinds to central stations. Fig. 253 shows one form. The number of different calls provided includes messenger, carrier, coupé, express wagon, doctor, laborer, police, fire, together with three more, which may be made special to suit the convenience of the individual customer. The instruments are provided with apparatus for receiving a return signal, the object of which is to notify the subscriber that his call has been received and is having attention. [Illustration: Fig. 254.] [Illustration: Fig. 255.] Fig. 254 shows another form of call box, the handle being moved around to the call desired. As it springs back to the original position, an interrupted current passes through the box to the central station, causing a bell to tap a certain number of times, giving the call and location of the box. =174. Electric Gas-Lighters.= Fig. 255 shows a _ratchet burner_. The first pull of the chain turns on the gas through a four-way gas-cock, governed by a ratchet-wheel and pawl. The issuing gas is lighted by a wipe-spark at the tip of the burner. Alternate pulls shut off the gas. As the lever brings the attached wire A, in contact with the wire B, a bright spark passes, which ignites the gas, the burner being joined with a battery and induction or spark coil. _Automatic burners_ are used when it is desired to light gas at a distance from the push-button. Fig. 256 shows one form. Two electromagnets are shown, one being generally joined to a white push-button for turning on the gas and lighting it, the other being joined to a black button which turns off the gas when it is pressed. The armatures of the magnets work the gas-valve. Sparks ignite the gas, as explained above. [Illustration: Fig. 256.] [Illustration: Fig. 257.] =175. Door Openers.= Fig. 257 shows one form. They contain electromagnets so arranged that when the armature is attracted by the pushing of a button anywhere in the building, the door can be pushed open. =176. Dental Outfits.= Fig. 258 shows a motor arranged to run dental apparatus. The motor can be connected to an ordinary incandescent light socket. In case the current gives out, the drills, etc., can be run by foot power. [Illustration: Fig. 258.] =177. Annunciators= of various kinds are used in hotels, factories, etc., to indicate a certain room when a bell rings at the office. The bell indicates that some one has called, and the annunciator shows the location of the call by displaying the number of the room or its location. Fig. 259 shows a small annunciator. They contain electromagnets which are connected to push-buttons located in the building, and which bring the numbers into place as soon as the current passes through them. [Illustration: Fig. 259.] INDEX. Numbers refer to paragraphs. See Table of Contents for the titles of the various chapters. Action of magnets upon each other, 32. Adjuster, for lamp cords, 151.