s 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]

Other types of switch will be found very useful. In Fig. 45 is a simple
sliding switch in which the base block has, at one end, a pair of copper
plates (A, B), each held at one end to the base by a binding screw (C),
and having a bearing or contact surface (D) at its other end. At the
other end of the base is a copper plate (E) held by a binding screw (F),
to the inner end of