nument 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 coke
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.

[Illustration: _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 s