Page:Popular Science Monthly Volume 92.djvu/167

 Electrical Devices and How They Work

Primary Battery Cells — I.

This article is the first of a series on electricity, each one of which is complete in itself. Some interesting experimen- tal problems are illustrated and very explicitly described

By Peter J. M. Clute, B. E.

��THE agency which comes into action when a circuit containing an electro- motive force is closed is called electric current. This current flow is analogous to the flow of water in pipes, or over the surface of the earth. Such a flow of water takes place only when from any cause, a difference in pressure exists between two points, or when the water is at different levels. When either of these conditions exists, the flow takes place in a certain direction; namely, from the higher to the lower level, the amount of flow being dependent upon the obstacles in its path. Electric currents, likewise, flow only in obedience to elec- trical pressure, and the quantity of cur- rent flowing is dependent upon the re- sistance to the flow offered by the circuit.

A simple primary cell is shown in Fig. 1. It consists of a glass jar nearly filled with a dilute sulphuric acid solu- tion, into which are placed a plate of zinc and one of copper. While the ends of the. wires connected with the plates remain separated, no current flows; but an electrical pressure exists, as can be readily shown. As soon as the ends of the two wires are brought into contact, a flow of current commences. It is the high resistance of the air between the two terminals which prevents the flow of current in this case, just as the presence of a closed valve in a waterpipe prevents water flow.

The current itself cannot be seen as it flows through the wire, but its effects are evidences of its presence. A current flowing through a thin wire will heat it; flowing through water and other liquids it decomposes them; flowing near a mag- netic needle it will cause it to deflect. All these phenomena cease at once when the current is interrupted, either by breaking the circuit or by separating the acid around one plate from that

��around the other by a non-conducting partition.

The direction of current is said to be from the zinc to the copper inside the cell and from copper back to zinc in the external circuit. In all cells the plate, or terminal, from which the current flows, is called the positive pole, and the terminal toward which the current flows in the circuit is the negative pole.

From the cell shown, Fig. 1, which is the simplest of all forms, a very insignifi- cant current is given. If several cells

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The simplest form of battery, the primary cell, and the gravity cell with wiring diagrams

are coupled together, as conventionally indicated in Fig. 3, 4 and 5, a consider- able current is obtainable. In this repre- sentation, the long thin lines indicate the positive plates and the short thick lines the negative plates.

The term "battery" is applied to a number of cells grouped together either in a series or in parallel and should never be applied to a single cell. Representa-

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