Testing
It would now be as well to test what you have already done. Connect the loose ends of the coil to the poles of a battery, reverse them, and repeat this two or three times. If the instrument has been properly made, the needle should swing to the left when the wires are connected in one way; to the right when they are reversed.
Fig. 4.—The keyboard.
Nothing more remains to be done but to make the keyboard, which is really the base of the instrument you have just been constructing. A glance at [Fig. 4] shows how this is to be done. C and D are two thin brass springs, 5 inches long, and, as they have to pass through the notches in the dial, less than half an inch in width, they must be bent upwards, so that when fastened to the board at C and D the opposite ends will spring up and press against the under side of the metal bridge BF. They are then in what is called “metallic contact.”
The bridge should be made from a piece similar to the springs and bent as shown in [Fig. 5]. Another strip of brass must be screwed flat to the board at AE, parallel to BF, and over this a couple of wooden or ivory knobs should be fastened to the springs, as they are used to press down CD in contact with AE ([Fig. 4]). When released they should fly back to BF. Be very careful to see that all these details are thoroughly in order, as they form the most vital part of the keyboard.
Fig. 5.—The instruments ready for use.