Ammonium chloride 1 part Zinc chloride 1 part Manganese binoxide 1 part Granulated carbon 1 part Flour 1 part Plaster of Paris 3 parts Water 2 parts These proportions may be measured in a tin cup, a table-spoon, or any other small receptacle. Note that the measurement by parts is always by bulk and not by weight. Do not fill the zinc cup to the top, but leave an inch of space, so that half an inch of sealing material may be added. See that the inside top edge of the zinc cup is clean; then melt some tar or pitch and pour it over the top of the electrolyte, so that it binds the zinc cup and carbon into a solid form. Drive an awl down through the capping material when it is nearly dry, and leave the holes open for the escapement of gases. Give the outer surface of the zinc cells a coat of asphaltum varnish, and wrap several thicknesses of heavy paper about them to prevent contact and short-circuiting. Protect the bottoms in a similar manner, and as a result you will have a cell that will appear as shown in Fig. 18 B. A battery of cells powerful enough for any light work can be made by connecting the cells in series, each having an electro-motive force of one and a half volts, with an internal resistance of less than one-third of an ohm. Chapter III PUSH-BUTTONS AND SWITCHES Push-buttons Push-buttons and switches are a necessity in every home where electric bells, lights, or fans are used, for with them connections are made or broken. The telegraph-key and the commutators on a motor and dynamo are only improved forms of the push-button, and this simple little device is really an indispensable part of any electrical equipment. The simplest form of push-button is a bent piece of tin or thin sheet-metal screwed fast to a small block of wood, as shown in Fig. 1. Under the screw-head one end of a wire is caught, and the other wire end is secured by a washer and a screw driven into the block directly under the projecting end of the strip of metal. By pressing a finger on the tin it is brought into contact with the screw-head under it, and the circuit is closed; on releasing it, the tin flies up and the circuit is opened again.