The stoppage or diminution of the current on the appearance of the gas upon the copper is attributed to what is called "polarization." It was first distinctly described by Ritter, who found that when two platinum wires were immersed in water, and a current was passed from one to the other, so that oxygen appeared at one of them and hydrogen at the other, if they were disconnected from the battery and joined through a conductor, a current in this new circuit would exist for a short time, the two wires acting like the plates of a battery. But in this case the direction of the current was opposite to that of the original circuit, showing that the electromotive force of what he called the "secondary" battery was opposite to that of the battery from which it was charged. He found that a Volta's pile might be constructed out of disks of the same metal; and, while it would not of itself exhibit any electrical properties, after it had formed a part of a circuit containing a battery of an ordinary form for a short time, it became an independent source from which a current could be drawn. This was the beginning of secondary or "storage batteries, to which so much attention has been given during the past few years.

A similar condition of things exists in the simple cell described. The presence of the

hydrogen at the copper plate produces polarization; that is, an electromotive force is set up, which, being opposed to that of the original combination, tends to neutralize the latter, and thus the available electromotive force of the cell rapidly diminishes. Innumerable devices

have been suggested for overcoming this difficulty, and a few of them have been practically successful.

So desirable, and, indeed, so essential to all practical applications of the electric current, was the construction of a battery which should be sensibly constant in its action for a considerable period of time, that the invention, about 1836, by John Frederick Daniell, of the wellknown battery which bears his name, must be regarded as an epoch in the history of the progress of electricity. The Copley medal of the Royal Society was well bestowed for the discovery of a device which rendered electrical experimentation comparatively easy from that time on; and, besides, it is the parent of nearly all useful later forms. The merit of the invention consists in the use of two liquids, suitably chosen, and their separation by means of a porous jar or diaphragm, which, being moist, does not offer injurious resistance to the passage of the current.

Daniell's battery, the still surviving parent of so many modern forms, is worthy of illustra

tion and explanation as the type of a large class. It consists essentially of an exterior vessel of glass or copper containing a solution of sulphate of copper, and, if the vessel be of glass,

a copper plate, which forms one of the poles. Within this is placed a porous cup or jar containing dilute sulphuric acid, in which the zinc pole is immersed. When in operation, metallic copper from the sulphate of copper is deposited

on the copper plate, which thus remains unchanged in its character. The solution of sulphate of copper is thus rendered weaker; but deterioration from this cause is provided against by depositing in the exterior jar, in any convenient manner, a mass of crystals of the sulphate, which, by gradually entering into the solution, maintains its strength. Sulphate of zinc is formed in the porous cup, and, when this comes to be excessive in amount, it must be removed and water substituted. The electromotive force of this battery is not as high as that of some others: when first put in operation as described, it steadily runs down until the dilute acid is saturated with sulphate of zinc, after which it remains constant for a very long time. With very little care and attention, it will work well for weeks, and, indeed, until the zinc pole is consumed.

It will be seen that this battery satisfies fairly well at least two of the requirements enumerated in the beginning. Concerning resistance, it may be said that the liquids used are moderately good conductors, and by increasing the size of the plates its resistance may be lowered to any desirable extent. Other forms, however, show much less resistance than the ordinary Daniell, and are to be preferred for certain classes of work. The fourth requirement refers

to useless expenditure of energy through chemical action, which goes on when no current of electricity is being generated. This is called "local action." If the zinc be perfectly pure, it will not take place. Whenever it occurs, it means that zinc is being consumed with no useful effect in the production of a current. It was long ago ascertained that impure zinc may be made to act, in this respect, as if it were pure, by amalgamating the surface exposed to the action of the acid. This is readily done by dipping it into dilute sulphuric acid, and afterward rubbing mercury upon it. When zinc is treated in this way, the action of the acid upon it is confined to the time during which the current is flowing.

Grove's and Bunsen's batteries are wellknown forms, in which polarization is prevented by chemical action, as in Daniell's cell, and the same principle is applied in nearly all forms of constant batteries. A few widely used forms, of which the well-known Leclanché is a type, are especially adapted for what is called "opencircuit" work, in which the current is never allowed to flow continuously for any length of time. For discontinuous service they offer many advantages.

The modern "gravity batteries," used so extensively in this country, are really forms of