Fig. 53 shows a _Table Clamp-Push_ for use on dining-tables, card-tables, chairs, desks, and other movable furniture. Fig. 54 shows a combination of push-button, speaking-tube, and letter-box used in city apartment houses. Fig. 55 shows an _Indicating Push_. The buzzer indicates, by the sound, whether the call has been heard; that is, the person called answers back. [Illustration: Fig. 53.] [Illustration: Fig. 54.] _Modifications_ of ordinary push-buttons are used for floor push-buttons, on doors, windows, etc., for burglar-alarms, for turning off or on lights, etc., etc. (See "Apparatus Book," Chapter III., for home-made push-buttons.) [Illustration: Fig. 55.] =62. Switches= have a movable bar or plug of metal, moving on a pivot, to make or break a circuit, or transfer a current from one conductor to another. Fig. 56 shows a _single point switch_. The current entering the pivoted arm can go no farther when the switch is open, as shown. To close the circuit, the arm is pushed over until it presses down upon the contact-point. For neatness, both wires are joined to the under side of the switch or to binding-posts. [Illustration: Fig. 56.] Fig. 57 shows a _knife switch_. Copper blades are pressed down between copper spring clips to close the circuit. The handle is made of insulating material. _Pole-changing switches_, Fig. 58, are used for changing or reversing the poles of batteries, etc. Fig. 59 shows a home-made switch, useful in connection with resistance coils. By joining the ends of the coils A, B, C, D, with the contact-points 1, 2, 3, etc., more or less resistance can be easily thrown in by simply swinging the lever E around to the left or right. If E be turned to 1, the current will be obliged to pass through all the coils A, B, etc., before it can pass out at Y. If E be moved to 3, coils A and B will be cut out of the circuit, thus decreasing the resistance to the current on its way from X to Y. Current regulators are made upon this principle. (See "Apparatus Book," Chapter IV., for home-made switches.) [Illustration: Fig. 57.] [Illustration: Fig. 58.] [Illustration: Fig. 59.] _Switchboards_ are made containing from two or three to hundreds of switches, and are used in telegraph and telephone work, in electric light stations, etc., etc. (See Chapter on Central Stations.) Fig. 60 shows a switch used for incandescent lighting currents. [Illustration: Fig. 60.] [Illustration: Fig. 61.] =63. Binding-Posts= are used to make connections between two pieces of apparatus, between two or more wires, between a wire and any apparatus, etc., etc. They allow the wires to be quickly fastened or unfastened to the apparatus. A large part of the apparatus shown in this book has binding-posts attached. Fig. 61 shows a few of the common forms used. (See "Apparatus Book," Chapter V., for home-made binding-posts.) CHAPTER VI. UNITS AND APPARATUS FOR ELECTRICAL MEASUREMENTS.