a devised what is known as the voltaic pile
(Fig. 20).

VOLTAIC PILE--HOW MADE.--This is made of alternate discs of copper and
zinc with a piece of cardboard of corresponding size between each zinc
and copper plate. The cardboard discs are moistened with acidulated
water. The bottom disc of copper has a strip which connects with a cup
of acid, and one wire terminal (A) runs therefrom. The upper disc, which
is of zinc, is also connected, by a strip, with a cup of acid from which
extends the other terminal wire (B).

[Illustration: _Fig. 20._ VOLTAIC ELECTRICITY]

_Plus and Minus Signs._--It will be noted that the positive or copper
disc has the plus sign (+) while the zinc disc has the minus (-) sign.
These signs denote the positive and the negative sides of the current.

The liquid in the cells, or in the moistened paper, is called the
_electrolyte_ and the plates or discs are called _electrodes_. To define
them more clearly, the positive plate is the _anode_, and the negative
plate the _cathode_.

The current, upon entering the zinc plate, decomposes the water in the
electrolyte, thereby forming oxygen. The hydrogen in the water, which
has also been formed by the decomposition, is carried to the copper
plate, so that the plate finally is so coated with hydrogen that it is
difficult for the current to pass through. This condition is called
"polarization," and to prevent it has been the aim of all inventors. To
it also we may attribute the great variety of primary batteries, each
having some distinctive claim of merit.

THE COMMON PRIMARY CELL.--The most common form of primary cell contains
sulphuric acid, or a sulphuric acid solution, as the electrolyte, with
zinc for the _anode_, and carbon, instead of copper, for the _cathode_.

The ends of the zinc and copper plates are called _terminals_, and while
the zinc is the anode or positive element, its _terminal_ is designated
as the positive pole. In like manner, the carbon is the negative
element or cathode, and its terminal is designated as negative pole.

Fig. 21 will show the relative arrangement of the parts. It is customary
to term that end or element from which the current flows as positive. A
cell is regarded as a whole, and as the current passes out of the cell
from the copper element, the copper terminal becomes positive.

[Illustration: _Fig. 21._ PRIMARY BATTERY]

BATTERY RESISTANCE, ELECTROLYTE AND CURRENT.--The following should be
carefully memoriz