Part 13

To obtain electricity from the dynamo we must spend money for the coal to make the steam which operates the steam-engine, or for the water which turns the water-wheel, as well as for an engineer in both cases. When we obtain electricity from a battery we must spend money for the chemicals and metals which are constantly consumed in the battery.

PRIMARY BATTERIES

An electrical battery is a device in which one or more chemical substances act upon a metal and a carbon, or upon two different metals, producing thereby a current of electricity, which will continue as long as there is any action of the chemicals upon the metal and carbon, or upon the two metals.

Batteries for producing electricity may be divided into two classes, called "open circuit" batteries and "closed circuit" batteries.

Open-circuit batteries are those which are used where the electricity is not required constantly without intermission--for instance, in telephones, electric bells, burglar alarms, gas-lighting, annunciators, etc.

Closed-circuit batteries are those which are used where the effect produced must be continuous every moment, as, for instance, in electric lights and motors.

The open-circuit battery is made in many different ways, so we only describe two of the principal ones.

As we told you in an early part of this book, we do not know just what electricity is, nor why it is produced under the conditions existing in a battery. But we do know that by following certain processes and making certain chemical combinations we can make as much electricity and in such proportions as we want.

The two metals, or the metal and carbon, in a battery are called the "elements," and to these are connected the wires which lead from the battery to the instruments to be worked by it.

The Leclanché Battery.--This form of open-circuit battery consists of a glass jar in which is placed the elements. One element consists of a rod of zinc, and the other element is carbon and powdered black oxide of manganese. These two (the carbon and black oxide of manganese) are placed in an earthenware vessel called a "porous cup." This is simply a small jar made of clay which is not glazed. Thus, the liquid which is in the glass jar penetrates through the porous cup to the carbon and manganese which it contains, and so the chemicals affect both these and the zinc at once, for, in order to obtain electricity, you will remember that the chemical action must take place at the same time upon both the elements in the same vessel. (Fig. 29.)

The chemical substance used in this battery is sal-ammoniac, or salts of ammonia. A certain quantity of this salt is dissolved in water, and this solution is poured into the glass jar. When this is done the battery will generate electricity at once.

It should be remembered that the proper term for the chemical mixture which acts upon the elements in any battery is "electrolyte."

The Dry Battery.--The cleanliness, convenience, high efficiency, and comparatively low internal resistance of the dry cell has brought it into great favor in the last few years. It is now extensively used in preference to the Leclanché and other open-circuit batteries having liquid electrolyte for light work, such as bells, gas-lighting, burglar alarms, ignition on motor-boats, automobiles, etc.

The dry cell is also used in great numbers for pocket flash-lamps, and in other ways where it would be impossible to employ batteries containing liquids.

A dry cell consists of zinc, carbon, and the electrolyte, which is a mixture so made that it is in the form of a gelatinous or semi-solid mass, so that it will not run or slop over.

A piece of sheet zinc is formed into a long tube, and a round, flat piece of zinc is soldered at one end, thus making a cup open at one end. This forms the cell itself, and at the same time becomes one of the elements. The other element is a piece of battery carbon which is long enough to project out of the top of the cell about half an inch or more. While the cell is being filled with the electrolyte the carbon is held up by a support so that it does not touch the zinc at the bottom of the cup. Of course, the zinc cup and the carbon are provided with proper binding-posts or other attachments, so that conducting wires can be connected.

The electrolyte is packed into the cup and around the carbon in such a way that the cup is entirely filled within about half an inch from the top, and then some melted tar or pitch is poured over the top of the electrolyte. This seals the cell and binds the contents solidly together. Just before the sealing compound hardens, one or two holes are made in it so that the gases may escape.

The composition of the electrolyte itself is not exactly alike in all dry cells, as the various manufacturers follow their own particular formulas. However, as you may be curious to know something about it, we would state that one formula embraces flour, water, plaster of Paris, granulated carbon, zinc chloride, ammonium chloride, and manganese binoxide.

You will remember that the Leclanché and the dry batteries are purely open-circuit cells, and that they can be used to advantage for electric bells, annunciators, burglar alarms, gas ignition, etc., where the current of electricity is not doing continuous work, but only for a few seconds at a time. Consequently, the batteries have a little rest in between, if only for a few seconds.

Now, if we were to attempt to use open-circuit batteries for electric lights or motors, where the electricity must work constantly every second, the batteries would "polarize"--that is to say, they would only work a few minutes and then stop, because the chemicals used in them are of that kind that they will only allow the battery to do a little work at a time.

The batteries we have been describing will do the ordinary work for which they are intended for sometimes a year without requiring any attention, but if we try to make them do work for which they were not intended, they would only last a few days.

If we should want to operate electric lights or motors continuously from a battery we must, therefore, use

CLOSED-CIRCUIT BATTERIES

There is a great variety of ways in which closed-circuit batteries are made, but, as the main principles are very much alike, we will only describe two general kinds, those with and those without a porous cup.

In the first place, we must state that closed-circuit batteries proper usually consist of a glass jar and two elements--carbon and zinc. Sometimes a porous cup is used; for what reason you will soon learn.

The chemicals that are used are usually different from those used in the open-circuit batteries and are much stronger. These chemicals are usually sulphuric acid and bichromate of potash (or chromic acid), which are mixed with water.

We will now examine two of the types of closed-circuit batteries, taking first the one without the porous cup, of which the Grenet is a good example.

This battery, as you see, consists of a glass jar, in which are placed two plates of carbon and one of zinc. (Fig. 30.) The latter is between the two carbon plates and is movable up and down, so that it may be drawn up out of the solution when it is not desired to use the battery. When the zinc is in the solution there is a steady and continuous current of electricity developed, which can be taken away by wires from the connections on top of the battery.