Part 14

If the zinc were left in the solution when the battery was not in use, the acid would act upon it almost as much as though the electricity were not being used, and thus the zinc would be eaten away and the acid would be neutralized, so that no more action could be had when we wanted more electricity.

Now, in the Grenet battery we can light a lamp or run a motor for several hours continuously, but at the end of that time the solution would become black and it would do no more work. Then we must throw out that solution and put in fresh, and the battery will do the same work again, and so on.

If you should only want to light your lamp or run your motor for a few minutes, you could pull the zinc up from the solution and put it down again when you wanted the electricity once more. The carbon element in the battery is not consumed by the acid, although the zinc is.

Now you will see the use of the porous cup. We will take as an illustration of this type an ordinary battery in which a porous cup is used. (Fig. 31.)

Here, you will see, the carbon is placed in the porous cup, while the zinc is outside in the glass jar. In the glass cell with the zinc is usually used water made slightly acid, and the strong solution of sulphuric acid and bichromate of potash (or chromic acid) is poured in the porous cup, where the carbon is placed.

The strong solution penetrates the porous cup very slowly and gets to the zinc, when it immediately produces a current of electricity. But the acid does not get at the zinc so freely as it does in the battery without a porous cup, and, consequently, neither the acid nor the zinc is so rapidly used up.

Where porous cups are used, the batteries will give a continuous current for a very much longer time than without them, and will, sometimes, give many hours' work every day for several months without requiring any change of solution.

Polarization.--There is one other reason why a longer working time can be had from a battery with a porous cup, and that is, in a battery without a porous cup the action of the acid upon the zinc is so rapid that the carbon plates become covered with gas, and, therefore, the proper action by the acid cannot take place upon them. Thus, the battery ceases to work, and is said to be "polarized." When a porous cup is used, the action of the acid upon the zinc is slow enough to give off only a small amount of gas, and thus the acid has a chance to act upon the carbon plates and develop a steady current of electricity.

THE WORK DONE BY BATTERIES

The pressure and quantity of electricity given off continuously by open and closed circuit batteries is very different.

The pressure (or "electromotive force") of one cell of an ordinary open-circuit battery is only about one volt, and the current is usually very much less than one ampère, except in a dry cell, which may give more.

In the closed-circuit batteries described, the electromotive force of each cell is about two volts, while the current varies from 1 to perhaps 50 ampères, according to the size of the zinc and carbon plates.

It would not matter if you made one cell as big as a barrel, nor if you put in a dozen carbons and zincs, the electromotive force would not exceed the volts mentioned for each type of battery, but the ampère capacity would be greater than in a smaller cell on account of the larger size of the carbon and zinc plates.

Internal Resistance.--There is one other point which affects the number of ampères which can be obtained from a closed-circuit battery, and that is whether there is a large or small internal resistance in the battery itself.

This depends upon the solution which is used and the arrangement of the plates.

If there is a high resistance in the battery itself (called "internal resistance"), the electricity must do work to overcome this resistance before it can get out of the battery to do useful work through the wires, and, consequently, the capacity in ampères is limited.

If, on the other hand, there is very little resistance in the battery, the current has very little work to flow to the wires leading from the battery, and we can get a larger quantity, or greater number of ampères.

Thus, you will see that while the closed-circuit battery is the stronger, and will do all that the open-circuit battery will do, and even more, in a short time the latter, though weaker, will do about as much work for the same amount of zinc and carbon as the former, but takes a much longer time.

BATTERIES FOR ELECTRIC LIGHT

As we have explained to you, closed-circuit batteries are used for producing incandescent electric lights in small numbers, as well as for running motors.

To operate incandescent lights, a number of batteries connected together are used. The number used depends upon the pressure which the lamps require to make them give the required light. We will now explain how the batteries are connected together for this purpose.

Suppose you wished to light an incandescent lamp of, say, three candle-power, which required six volts. We would take three closed-circuit batteries which would each give two volts, and connect by a piece of wire the zinc of the first to the carbon of the second, and the zinc of the second to the carbon of the third, as shown in the sketch. (Fig. 32.)

We would then attach a wire to the carbon of the first and one to the zinc of the third, and there would be six volts in these two wires, which would light up one six-volt lamp nicely.

This is called connecting in series, or for intensity.

Now if each of these cells gave ten ampères alone, the three will only give ten ampères together when they are connected in series.

If our lamp only required one ampère, you would naturally think that ten similar lamps put on the wires would give as good light as the one, but that is not so.

Although you might light up two lamps, the pressure would drop and the lights would become less brilliant if you put on the whole number. So, if we wished to put on the whole ten lights we would connect another battery and thus increase the pressure, which would probably make these ten lamps burn brightly.

These rules hold good for connecting any number of batteries for lamps of any number of volts--that is to say, there should be calculated about two volts for each cell and an allowance made for drop in pressure.