THE VOLTAGE STANDARD.--In order to determine the voltage we must know something of _resistance_. Different metals do not transmit a current with equal ease. The size of a conductor, also, is an important factor in the passage of a current. A large conductor will transmit a current much better than a small conductor. We must therefore have a standard for the _ohm_, which is the measure of resistance. THE OHM.--It is calculated in this way: There are several standards, but the one most generally employed is the _International Ohm_. To determine it, by this system, a column of pure mercury, 106.3 millimeters long and weighing 14.4521 grams, is used. This would make a square tube about 94 inches long, and a little over 1/25 of an inch in diameter. The resistance to a current flow in such a column would be equal to 1 ohm. CALCULATING THE VOLTAGE.--In order to arrive at the voltage we must use a conductor, which, with a resistance of 1 ohm, will produce 1 ampere. It must be remembered that the volt is the practical unit of electro-motive force. While it would be difficult for the boy to conduct these experiments in the absence of suitable apparatus, still, it is well to understand thoroughly how and why these standards are made and used. CHAPTER VII PUSH BUTTONS, SWITCHES, ANNUNCIATORS, BELLS AND LIKE APPARATUS SIMPLE SWITCHES.--We have now gone over the simpler or elementary outlines of electrical phenomena, and we may commence to do some of the practical work in the art. We need certain apparatus to make connections, which will be constructed first. A TWO-POLE SWITCH.--A simple two-pole switch for a single line is made as follows: A base block (A, Fig. 43) 3 inches long, 2 inches wide and 3/4 inch thick, has on it, at one end, a binding screw (B), which holds a pair of fingers (C) of brass or copper, these fingers being bent upwardly and so arranged as to serve as fingers to hold a switch bar (D) between them. This bar is also of copper or brass and is pivoted to the fingers. Near the other end of the base is a similar binding screw (E) and fingers (F) to receive the blade of the switch bar. The bar has a handle (G) of wood. The wires are attached to the respective binding screws (B, E). DOUBLE-POLE SWITCH.--A double-pole switch or a switch for a double line is shown in Fig. 44. This is made similar in all respects to the one shown in Fig. 43, excepting that there are two switch blades (A, A) connected by a cross bar (B) of insulating material, and this bar carries the handle (C). [Illustration: _Fig. 43._ TWO-POLE SWITCH] [Illustration: _Fig. 44._ DOUBLE-POLE SWITCH]