It will be noted from the sketch that the zinc plate is a little longer than the carbon. As it is advisable to remove the plate from the battery when not in use, the extra length of the zinc plate will protect the carbon to some extent from breaking by frequent handling. A very convenient arrangement for a battery of this sort is to mount the cells on a wooden base, which has at either end an upright post, about 10 or 12 inches high. A rod or a shaft may be fitted in the top of the post and provided with a crank or hand-wheel so as to form a sort of windlass. The crossbars of each cell may all be suspended from a strip of wood which is connected to the shaft by a small chain or cords, and a catch or ratchet wheel should be fitted on the shaft, so that when the battery is not required for use the plates may all be raised above the jars by winding up the supporting chain or cord, and when wanted they may be instantly lowered into the solution. Another arrangement sometimes used is to have a circular wooden base with a post in the center, this post carrying a sliding support to which the plates may be connected.

While the single cell battery just described is perhaps the most convenient for the amateur, owing to ease of construction, compactness and strength, it is not used to any extent in commercial applications. Such cells may be divided into two types, the open and closed circuit cells. The first is adapted only for intermittent work, such as the operation of electric bells, annunciators, and all other devices requiring a momentary current. The form of open circuit cell used almost universally for this work is the Leclanche (Fig. 5) or some of its modifications. The elements are zinc and carbon, the zinc being in the shape of a rod or cylinder. The carbon in the original form consists of a plate mounted in a porous cell of unglazed earthenware surrounded by a mixture of black oxide of manganese and broken carbon, the top of the cell being sealed up with insulating compound. It is a single fluid cell, using a saturated solution of sal ammoniac. In the more recent forms, the porous cell is dispensed with and the negative consists usually of a carbon cylinder only. Dry batteries, now so largely used, are of this type, but instead of the solution of sal ammoniac, a paste is used and the entire cell is sealed up. Such a cell can be used in any position, and as there is no evaporation it is a very convenient form for light work, but cannot be renewed conveniently when exhausted.

The closed circuit cell is made in several forms, the most familiar being the gravity or sulphate of copper cell used largely for telegraph work (Fig. 6) and described in the last number of this magazine. Another cell largely used for operating telephone transmitters is known as the Fuller. The elements in this cell are known as zinc and carbon. The solution is a bichromate of potash in water to which sulphuric acid has been added. The zinc is cone shaped and placed in the bottom of a porous cell containing mercury and filled with water. The bichromate solution is outside of the porous cell surrounding the carbon. The mercury within the porous cell keeps the zinc constantly amalgamated. The pressure or electromotive force of this cell is only two volts, and it requires but little attention to keep it in good working order. Another well-known single fluid cell is the Grove, using zinc and platinum with a solution of nitric acid and water. The Bunsen cell differs from this only in the substitution of carbon for platinum. Both of these cells have a pressure of about 1.03 volts. There is practically no limit to the combination of metals and exciting solutions which may be used to form a galvanic battery. The above list, however, covers the forms most largely in use at the present time. An alkali solution may be used instead of acid in a battery, and a cell of this type, quite well known, is the Edison - Laland battery, which consists of a plate of oxide of copper for one electrode and zinc for the other. A caustic potash solution is used, which is covered by a layer of heavy oil to prevent evaporation. The electromotive force of this cell is quite low, - about .8 volt, - but as there is no action when on open circuit and the internal resistance of the cell is low, this form of cell is well adapted for operating small motors and is frequently used for supplying a current for phonographs and slot machines. Before leaving the subject of batteries, the student's attention is directed to the phenomena of polarization, a defect peculiar to nearly all forms of galvanic batteries. This defect may be well shown and easily investigated by taking a single cell of the bichromate battery described and filling the jar with a weak solution of sulphuric acid and water. Now if the two plates are connected by means of a wire, a current is at once developed. In a few moments, bubbles of gas will be noticed on the surface of the two plates, and the current and the strength of the battery will diminish rapidly as the accumulation of gas increases. The cause of this failure of the current depends not only on the weakening of the solution as the battery is in use, but also on the fact that the layer of hydrogen gas on the copper or carbon plate gradually increases the internal resistance of the •cell and so prevents the maximum now of current. In addition to this effect, the hvdrogen gas on the negative plate and the oxygen gas on the zinc develop a counter electromotive force or pressure in opposition to the current flowing in the circuit. This failure of the battery is called polarization. If the gas be partially removed from the plates by shaking the cell or brushing the plates, the strength of the current will rise. In practical use, however, the same result is arrived at by chemical means. In the gravity battery, the hydrogen gas decomposes the copper sulphate, depositing metallic copper on the copper or negative plate, and frees the sulphuric acid, which was part of the copper sulphate, thus continually renewing the surface of the copper and preventing the accumulation of gas. In the Grove and Bunsen batteries, the hydrogen is reduced to water by the nitric acid, thus keeping the negative element free. In the Leclanche type, the hydrogen is reduced to water by the oxide of manganese. In the bichromate cell, the chromic acid formed from the bichromate salt performs the same office. A rough means of showing the effect of polarization may be had by eonnecting an electric bell or the tele-graph sounder described in the November number of Amateur Work, including a coil of fine wire or other resistance sufficient to make the bell-sounder work feebly on the single cell with acid solution described. When the plates are first put in the jar, the bell or sounder will work quite strongly, but as polarization increases, the effect will become gradually weaker until finally the instruments will refuse to work.

Studies In Electricity II Batteries 31

Gravity Battery.

Studies In Electricity II Batteries 32

Leclanche Battery.