When the elements are connected at their terminals by a wire or conductor a chemical action takes place, producing a current which flows from the copper to the zinc. This device is called a _cell_, and the combination of two or more of them connected so as to form a unit is known as a _battery_. _The word battery is frequently used incorrectly for a single cell._ That terminal of the element from which the current flows is called the _plus_ or _positive pole_, and the terminal of the other element the _negative pole_.

Cells are said to be _primary_ or _secondary_ according as they generate a current of themselves, or first require to be charged from an external source, storing up a current supply which is afterwards yielded in the reverse direction to that of the charging current.

An electric current is generated mechanically by a _dynamo_. In either case _no electricity is produced, but part of the supply already existing is simply set in motion by creating an electric pressure_.

An electric current, according to the third method, is generated directly from heat energy, as will be later explained; the current thus obtained is very feeble.

[Ill.u.s.tration: FIG. 38.--Hydrostatic a.n.a.logy of fall of potential in an electrical circuit.]

[Ill.u.s.tration: FIG. 39.--Showing method of connecting voltmeter to find potential difference between any two points as _m_ and _n_ on an electrical circuit.]

=Strength of Current.=--It is important that the reader have a clear conception of this term, which is so often used. The exact definition of the strength of a current is as follows:

_The strength of a current is the quant.i.ty of electricity which flows past any point of the circuit in one second._

_Example._--If, during 10 seconds, 25 coulombs of electricity flow through a circuit, then the average strength of the current during that time is 2-1/2 coulombs per second, or 2-1/2 amperes.

=Voltage Drop in an Electric Circuit.=--A difference of potential exists between any two points on a conductor through which a current is flowing on account of the resistance offered to the current by the conductor.

For instance, in the electrical circuit shown in fig. 39, the potential at the point _a_ is higher than that at _m_, that at _m_ higher than that at _n_, etc., just as in the water circuit, shown in fig. 38, the hydrostatic pressure at _a_ is greater than that at _m'_, that at _m'_ greater than that at _n'_, etc. The fall in the water pressure between _m'_ and _n'_ (fig. 38) is measured by the water head _n's_.

In order to measure the fall in electrical potential between _m_ and _n_, (fig. 39), the terminals of a volt meter are placed in contact with these points as shown. Its reading will give the difference of potential between _m_ and _n_, in volts, provided that its own current carrying capacity is so small that it does not appreciably lower the potential difference between the points _m_ and _n_ by being touched across them; that is, provided the current which flows through it is negligible in comparison with that which flows through the conductor which already joins the points _m_ and _n_.

CHAPTER IV

PRIMARY CELLS

The word "battery" is a much abused word, being often used incorrectly for "cell," as in fig. 40. Hence, careful distinction should be made between the two terms.

_A battery consists of two or more cells joined together so as to form a single unit._

There are numerous forms of primary cell; they may be cla.s.sified as follows:

1. According to the service for which they are designed;

2. According to the chemical features.

With respect to the first method cells are cla.s.sified as:

1. Open circuit cells;

Used for _intermittent work_, where the cell is in service for short periods of time, such as in electric bells, signaling work, and electric gas lighting. If kept in continuous service for any length of time the cell soon polarizes or "runs down,"

but will recuperate after remaining on open circuit for some little time.

2. Closed circuit cells.

This type of cell is adapted to furnishing current continuously, as in telegraphy, etc.

With respect to the second method, cells are cla.s.sified as:

1. One fluid; 2. Two fluid;

=Ques. Describe a primary cell.=

Ans. A primary cell consists of a vessel containing a liquid in which two dissimilar metal plates are immersed.

In _one fluid_ cells both metal plates are immersed in the same solution. In _two fluid_ cells each metal plate is immersed in a separate solution, one of which is contained in a porous cup which is immersed in the other liquid.

=Ques. What name is given to the metal plates?=

Ans. They are called _elements_.

=Ques. What is the fluid called?=

Ans. The _electrolyte_ or _exciting fluid_.

The term "electropoion" is a trade name for the electrolyte employed in the Fuller cell.

=Action of a Primary Cell.=--The fundamental fact on which the electro-chemical generation of current depends is, that if a plate of metal be placed in a liquid there is a difference of electrical condition produced between them of such sort that the metal either takes a lower or higher electrical potential than the liquid, according to the nature of the metal and the liquid. If two different metals be placed in one electrolytic liquid, then there is a difference of state produced between them, so that, if joined by wire outside the liquid, a current of electricity will traverse the wire. This current proceeds in the liquid from the metal which is most acted upon chemically to that which is least acted upon.

Referring to fig. 41, the construction and action of a simple primary cell may be briefly described as follows:

Place in a gla.s.s jar some water having a little sulphuric or other acid added to it. Place in it separately two clean strips, one of zinc, Z, and one of copper, C. This cell is capable of supplying a continuous flow of electricity through a wire whose ends are brought into connection with the two strips. When the current flows, the zinc strip is observed to waste away, its consumption in fact furnishing the energy or electromotive force required to drive the current through the cell and the connecting wire. The cell may therefore be regarded as a kind of chemical furnace in which the fuel is the zinc.

[Ill.u.s.tration: FIG. 40.--Simple primary cell. It consists of two dissimilar metal plates (such as copper and zinc which are called the _elements_), immersed in the _electrolyte_ or exciting fluid contained in the gla.s.s jar.]

=Ques. How are the positive and negative elements of a primary cell distinguished?=

Ans. The plate attacked by the electrolyte is the negative element, and the one unattacked the positive element.

=Chemical Changes; Polarization.=--The chemical changes which take place in a simple cell, consisting of zinc and copper elements in an electrolyte of dilute sulphuric acid, may be briefly described as follows: When the two elements are connected and the current commences to flow, the sulphuric acid acts on the surface of the zinc plate and forms sulphate of zinc. The formation of this new substance necessitates the liberation of some of the hydrogen contained in the sulphuric acid, and it will be found that bubbles of free hydrogen gas speedily appear on the surface of the negative element, that is, on the copper plate.

While the zinc is being dissolved to form zinc sulphate, hydrogen gas is liberated from the sulphuric acid.

[Ill.u.s.tration: FIG. 41.--Simple primary cell with circuit closed, showing direction of the current.]

Some bubbles of the gas rise to the surface of the electrolyte and so escape into the air, _but much of it clings to the surface of the copper element which thus gradually becomes covered with a thin film of hydrogen_.

Partly on account of the decreased area of copper plate in contact with the electrolyte, and partly because the hydrogen tends to produce a current in the opposite direction, the useful electrical output becomes considerably diminished and the cell is said to be _polarized_. This state of affairs may be rectified by stirring up the electrolyte, or by shaking the cell, so as to a.s.sist the hydrogen bubbles to detach themselves from the surface of the copper plate and make their way to the atmosphere through the electrolyte. This, however, is only a temporary remedy, as the polarized condition will soon be reached again, and a further agitation of the cell will be necessary. Hence, a simple cell of this kind is not desirable for practical work, and it must be modified to adapt it to constant use.

When the sulphuric acid in a cell acts in the zinc element and produces sulphate of zinc, a certain amount of work is done which is manifested partly in the form of useful electric energy, and partly as heat which warms the electrolyte and which is thereby lost for all practical purposes.

=Ques. If the zinc and copper electrodes of a simple cell be not connected externally what changes take place within the cell?=

Ans. The zinc plate immediately becomes strongly charged with negative electricity, and the copper plate weakly so. As long as the plates remain unconnected, and the zinc is pure, no further action takes place.