"The sulphur that is in the samples of copper ore, if burned, will make a sulphurous acid gas, and while it must be carefully used, on account of its noxious and offensive odor, is a most powerful germicide. Or if we take some of the green acid of the copper, and make a liquid of it, and then pour this over common salt we are making what is known as muriatic acid. The vapor of this acid will destroy all germs. The objection to this, however, is, that it has an odor which is worse than the impure or unhealthful gases. In the last samples of ore we brought home, you may have noticed a very black lot of stuff. That was manganese. If we take the muriatic acid, which I have just referred to, and pour it over the manganese, we can make the most powerful agent of all, namely, chlorine."

"Chlorine is used for bleaching, isn't it?"

"That is its great use in the arts; but as a purifier it has no equal.

It will decompose every gaseous compound and evil-smelling gas which escapes from decayed animal or vegetable matter."

Harry did not let the Professor forget the matter of the primary electric battery which he had been making preparations for, and after they had gone over the sanitary features of their kitchen, he was anxious to make a start. George was equally insistent, because the question of a better light was ever uppermost in his mind.

"Then here goes for the primary battery. We haven't any gla.s.s, nor have we found any rubber lying around, so we can make cells out of them, so what shall we use?" was the Professor's first inquiry.

"Why not use some of these iron pots we made?"

"I am afraid iron would not last very long, with sulphuric acid in them.

We should use some non-conductor of electricity."

"What do you mean by non-conductor?"

"That quality in a material which will not allow electricity to pa.s.s."

"Then why can't we use clay? Will that conduct electricity?"

"That is just the thing. Isn't it wonderful what a friend common clay has been to us since we have been on this island!"

"I think we ought to erect a monument to Monsieur Clay?"

"It would be a fitting thing to do, because at the end we are all bound to go back to him."

"I believe you said, Professor, that we should have to grind up the c.o.ke and then mix it up and make plates out of it?"

"Yes; we can use either carbon or copper for the negative plates."

"What are the other plates?"

"The positive. That is what I wanted the zinc for, which we made several weeks ago."

"Why should we have positive and negative plates in a battery?"

"Everything must have an opposite. If there is an up there must be a down; there would be no darkness without light; no heat without cold; no strength without weakness, and no joy without sorrow. Like all these things, the electric current flows from one to the other."

"But in electricity the current flows only one way, does it not?"

"In the primary battery that is the case; but when electricity is generated and sent over the wires, the natural current flows in both directions--that is, it goes in one direction as much as in the other."

"I do not understand what you mean by that."

"The current alternates. What is meant by that is this: For an instant the current flows from the positive to the negative, and the next instant it flows from the negative to the positive, and so on, making the alternate current."

"Then the primary battery we are going to make will be another kind of current?"

"We shall make what is called the direct current which goes in one direction only--that is, within the battery it moves from the positive plate, the zinc, to the copper plate, or negative, and outside of the battery it moves from the negative to the positive plate."

"Why does it do so?"

"In order that you may understand, I shall make a drawing so Harry will not have so much trouble in arranging the parts. So if you will examine the sketch (Figure 25), you will see that the clay cell, which we are to make, has in it the two electrodes, A and B. That is what they are called when they are spoken of together; but the positive one (A), the zinc, is called the anode, and the negative (B), or copper, is called the cathode. You should keep these terms in mind.

[Ill.u.s.tration: _Fig. 25. PRIMARY BATTERY_]

"The liquid in the cell, marked C, is used as the electrolyte, and for that we shall take some of the sulphate of copper which the copper ore furnishes. A good strong salt solution would also answer the purpose.

The two electrodes are separated, and a wire connects the two outside of the cell. Now you will notice that within the cell the current flows, as shown by the dart E, from the positive to the negative plate, but outside of the battery the current flows through the wires F from the negative to the positive plate."

"I can understand it now. The current from the battery will always go from the negative to the positive pole."

"You are mistaken. I am glad you referred to that. It shows the importance of using correct terms. You must not confound the terms 'negative plate' with 'negative pole.' All currents leave the battery or dynamo from the negative plate, but that negative plate is called the positive pole of the dynamo."

"It seems to me that is a curious way to do it."

"Such is the case, however; but there is no real positive or negative in the alternating current, so that either side may be termed positive or negative."

Work on the battery continued for some days, as lack of fine tools made much of the work difficult, and in doing this work, as in everything else, a certain amount of preparation was necessary. They had no screws, and no facilities for making them, so a subst.i.tute had to be devised, but the difficult part now to encounter was the preparation of the wire.

"A battery is of no use unless we can have wire, and it will be a big job to beat out wire long enough for our purposes," Harry observed as the battery neared completion.

"Then we must draw some wire?"

"From what?"

"From the copper?"

"Is that better than iron?"

"Copper should be used for several reasons; first, because electricity travels through a copper wire more easily than through iron, and second, for the reason that copper is more ductile than iron, and can be drawn into a wire with greater facility."

"Doesn't electricity flow through different substances at the same rate of speed?"

"Yes; but it r.e.t.a.r.ds the amount or the force."

"You say, 'Amount' or 'Force.' I can understand that if applied to water, that there might be a large or small quant.i.ty of water, or a greater or less pressure, but I do not see how this applies to electricity."

"In measuring the pressure of water, calculation is made by taking the height of the water in the tank. For every 28 inches in height a column one inch square weighs one pound. This represents the force of the water when it issues from the orifice below. Now the orifice may be large or it may be small. The amount or quant.i.ty which flows out is dependent on the size of the opening. Electricity is measured in a somewhat similar manner. What is called 'Volts' is the same as the force in the tank--that is, voltage means the pressure. Amperage, on the other hand, refers to the amount of current which is pa.s.sing, and a greater quant.i.ty will pa.s.s over a large wire just the same as a greater amount of water will flow through a large than a small pipe. Is this perfectly clear to you?"

"Yes; I understand the difference, now."

The drawing of wire is not a difficult task where facilities are at hand, but it must be remembered that all their tools were of the crudest kind. Harry had prepared a number of bars of copper, each having been beaten out to form pieces about ten inches long and a half inch thick. A steel plate about three-eighths of an inch thick, two inches wide, and six inches long, had a number of holes bored through it, the largest hole being a half inch in diameter, and gradually increasing in size, the smallest being about a sixteenth of an inch in diameter.

[Ill.u.s.tration: _Fig. 26. Template for Drawing Wire._]

When all was ready Harry was instructed to hammer out one end, so it would go through the largest hole. The projecting end was then grasped by a pair of heavy pliers, and pulled through, so that the bar was formed the size and shape of the first hole, and of course the bar was lengthened. The end was then hammered out so that it would go through the next smaller hole, and the same process was repeated, and when the wire got larger they had a tool which pushed the wire in at the same time it was being pulled out at the other side.

It was laborious work, and a long time was consumed in fully drawing out each bar. In this way a quant.i.ty of serviceable wire was prepared.