FREDERICK A. DRAPER

The multiplicity of types and kinds of galvanic batteries and the variety of uses to which they may be adopted, is more or less confusing to the novice in electrical work. These chapters are written, therefore, for the information of those who contemplate steady and experimental work involving the construction and use of batteries, the examples selected being those best adapted to the work for which they were designed and which best lend themselves to construction by the in-experienced in such work.

As an introduction, it will be advisable to briefly present the principles which underlie the action of batteries in general, reserving detail mention of the action of each particularkind for the chapter in which it is described.

It may also be well to state at this time that a battery does not "generate " electricity, although for the purpose of convenient expression, it is frequently so stated. The function of a battery is to maintain a current of electricity through or along a conductor by means of the chemical action set up between two metals, immersed in a liquid possessing a chemical ** affinity" for one of them.

The energy with which this combination of metal and liquid takes place, determines the E. M. F. (electro motive force) peculiar to the particular metals and liquids used. Every element possesses a specific amount of latent or potential energy, knowledge of which enables us to calculate the E. M. F. to be obtained from a given combination of elements.

These combinations of elements are always in certain, fixed proportions, which has led to the assigning of numbers to each element, representing the proportion by weight, which the element forms of any compound. These numbers are known as the "Atomic Weights," and represent the relative weights compared with Hydrogen, the lightest of the elements, which has been taken as the unit.

As different combinations of the same elements form different substances, it is evident that elements must be capable of replacing one another. The weight of an element which will replace unit weight of another element is known as the "chemical equivalent," or combining weight. This may be the same or different from the ratio of the atomic weights of the two elements. The ratio of the atomic weights to the chemical equivalent is known as the "Yalency " of the element, and is also the number of atoms of hydrogen required to replace one atom of the element.

The substance formed by different combinations of elements are designated by symbols, those of particular interest in these chapters being shown in the following table, together with the atomic weights, chemical equivalents and valencies.

This arrangement is also in accord with the electrical condition of each element, electro positive to the ones below, and electro-negative to the ones above it. To illustrate what has been stated we will study the action which takes place in a very simple form of cell.

A strip of chemically pure zinc is placed in dilute sulphuric acid; no action follows. A piece of copper or carbon is also placed in the acid; still no action. The exposed ends of the zinc and copper are allowed to touch or are connected with a short piece of copper wire representing an external circuit; the acid immediately attacks the zinc, the latter is dissolved, zinc sulphate formed and hydrogen liberated in minute bubbles. This chemical action sets up a current from the zinc (anode) through the liquid (electrolyte) to the copper or carbon (cathode), carrying with it the hydrogen bubbles which collect upon the surface of the cathode. The function of the cathode is mainly that of a conductor of the current from the electrolyte.

The chemical action which takes place, as mentioned above, is as follows: The affinity of the electrolyte for the anode and resulting combination and changes has set up a difference of potential between the terminals (electrodes) of the cell, which is equalized by the current flowing through the external circuit. The current flowing through the electrolyte, breaks up the compounds previously formed, and restores in part the potential energy to the atoms, which again unite with the metal forming the anode maintaining the E. M. F. and a continuous flow of current. The current continues to flow only while the circuit is closed, and ceases as soon as the circuit is broken.

If, instead of chemically pure zinc, commercial zinc be used, action within the cell does not cease when the circuit is broken. Commercial zinc is not sufficiently refined to remove all the other metals usually associated with it in the ore, and the traces of copper, iron, etc., remaining therein, cause the chemical action to be continued, and zinc is consumed without doing useful work. This is known as " local action, " and is undoubtedly caused by the foreign metals which serve as minute cathodes for the flow of local currents.

If the action of this cell be continued by keeping the external circuit closed the hydrogen bubbles will not all rise to surface, but an increasing number will collect upon the surface of the cathode, interfering with the free flow of the current. As the hydrogen bubbles are electro-positive like the zinc, the cathode or negative plate is thus gradually converted into apos-itive one. This action is known as " Polarization, " and is a source of great loss of constancy in cells of certain types. As will be shown in later chapters, it is possible to select and arrange elements which will overcome this excessive tendency to polarization, while at the same time a current is flowing through the circuit. There are several ways of doing this, the one most commonly employed being the addition of some substance in the cell, with which the hydrogen gas will readily combine. Such substances are known as "depolarizers, " but the rate of their action is dependent upon several conditions. No depolarizer will maintain the E. M. F. of a cell constant with varying currents, the construction of the cell and its size having much to do with the amount of hydrogen liberated and the amount which can be absorbed by the depolarizer.

From what has been stated, it is evident that to obtain a cell giving a high E. M. F.,the metal chosen must be one, for which the electrolyte has great affinity, but this is limited by the necessity that little or no action occurred except during the passage of the current. For these reasons zinc is the metal most extensively used, as well as the fact that it is the cheapest metal, excepting iron.

By the consumption of zinc, then, do we obtain electrical energy. It requires but little calculation to show, however, that as a source of energy on a large scale for lighting or power purposes, the coat of materials, and maintainance is prohibitive in competition with dynamos which can supply current at a fraction of the cost from batteries, and far more conveniently.

For such uses as ringing bells, lighting gas or igniting engines or for laboratory experimental work a suitable battery is efficient and easily and cheaply maintained.