From the Electrical Review Fig

FROM THE "ELECTRICAL REVIEW" FIG. 7.-THREE SIEMENS DRY CELLS AND A BATTERY OF 36 CELLSFig. 7.-THREE SIEMENS DRY CELLS AND A BATTERY OF 36 CELLS expansion, and covered with a paper disc through which two small glass ventilating tubes protrude in the case of large cells and the cell is finally sealed with pitch.

An illustration of three standard sizes of cells is shown in fig. 7. The battery shown in fig. 7 is made of 36 cells, connected in series to give a nominal e.m.f. of 5o volts; its weight is 20 lb. and maximum discharge rate 20 milliamperes.

Chemical Reactions in the Dry Cell.

These are not ex actly understood, but the principal changes taking place at the electrodes may be briefly described as follows:— The zinc in contact with the solution of ammonium chloride becomes negatively charged because of the departure of positive zinc ions Zn • • from its surface. As zinc dissolves in the solution, zinc ions, ammonia and hydrogen ions are produced, according to the ionic equations:— The carbon-manganese dioxide electrode in contact with the solution of ammonium chloride becomes positively charged. This fact may be explained in at least two ways. According to the first theory, hydrogen ions (H• ) are discharged at the surface of the composite electrode and render it positive.

21-1• --*

+ 2 ®.

The manganese dioxide rapidly oxidizes the hydrogen, which would otherwise accumulate on the surface of the electrode and polarize the cell. The manganese dioxide is thereby re duced to a lower state of oxidation, probably 2Mn02 + H2 = Mn203 + H20.

According to the second theory, the manganese dioxide gives tetravalent ions (Mn ""), which are reduced during the ac tion of the cell to ions of a lower valency and thereby furnish positive charges to the electrode + G.

The manganese dioxide thus diminishes the polarization of the cell, and is at the same time reduced to a lower state of oxida tion. If the positively charged electrode (carbon-manganese dioxide) is connected with the negatively charged electrode (zinc) by a wire, a current will flow through the wire from the carbon to the zinc. Within the cell the current will flow from the zinc through the electrolyte to the carbon-manganese dioxide.

Recent Improvements.

These have been made mainly in the attainment of increase of amperage yield, capacity, the reduc tion of deterioration and the improvement in dependability. These advances have been secured by care in the selection of raw ma terials, close control of manufacturing methods and processes, and testing of the products.

Experiments conducted by G. W. Vinal and R. M. Ritchie show that dry cells deteriorate even though they are not in serv ice. The small cell wastes away more rapidly than the larger one. Deterioration, however, can be measurably retarded by storing the cells and battery in a cool, dry place. The user of radio ap paratus when employing dry cells should not allow them to freeze. Cells must not be tapped for excessive current requirements. A marked gain in capacity is obtained by making the drain on the current light.

Desiccated cells are manufactured dry and require the addi tion of water before they are ready for use. Some of them are manufactured as paper-lined cells and others are of the bag type. Each cell is provided with an opening in the seal or centre of the carbon rod, through which the water necessary to make the cell active may be introduced. Some of them are also provided with a vent. Only two kinds of these cells are well known in this country, but others are now being developed. One of these, called a "reserve" cell, closely resembles an ordinary dry cell. The other, called the "add water," is of bag-type construction with an inner zinc for the electrode. When in use it contains rather more electrolyte than the ordinary dry cell.

Action of Heat.

The effect of temperature on the open circuit voltage of dry cells is small and for most purposes can be neglected entirely. The temperature coefficient is positive within the ordinary range of temperatures; that is, an increase in temperature is accompanied by a rise in voltage.

Between o° and 25° C (32° and 77° F) the short-circuit cur rent increases by approximately i ampere for each 3° C (5.5 ° F) rise in temperature. At higher temperatures the rate of increase is somewhat less.

Heat produces deterioration of dry cells in two ways. First, it tends to produce leakage, this may be observed when the sticky electrolyte has oozed out around the seal of the cell. Second, it increases the rate of the chemical reactions taking place within the cell. The deterioration of the cells is usually measured by the decrease in the short-circuit current with time when the cells are stored on open circuit.

Testing of Cells.

Service tests, which are applied to gauge the efficiency of cells, include determinations of ampere-hour and watt-hour capacity at both constant current and constant resist ance discharge.

Since dry cells are mostly used on circuits of which the re sistance is constant or nearly so, the capacity is usually ex pressed as the number of hours or days that the cell will continue to give service on such a circuit. This period is calculated to the time that the terminal voltage has fallen to some arbitrary value below which the service is not satisfactory. For example, the cut off voltage of a group of three cells for telephone service is 2.8 volts. Such service is intermittent and extends over a number of months. The capacity of dry cells may also be expressed in terms of ampere hours and watt hours, but to obtain these data it is necessary to integrate carefully measured discharge curves. In any case the capacity, whether expressed as hours, ampere hours, or watt hours, depends on the condition of the cells, the way they are made and the arbitrary choice of the cut-off voltage.

Apart from the voltage on open circuit it is necessary to de termine the rate of exhaustion and the power of recuperation of the cell. In a method which has now been adopted practically as standard, the cell is discharged five minutes daily through a 4 ohm resistance until the e.m.f. falls to a value of o.75 volt per cell.

The Open-Circuit Voltage test is usually made with a volt meter through which some current necessarily flows. It is, there fore, not strictly an open circuit measurement, but the current which flows through the voltmeter is generally so small that the voltage of the cell is lowered by an amount which is negligible. An accurate voltmeter of at least ioo ohms resistance per volt of the scale and having at least 5o divisions per volt should be used for this purpose. The true open-circuit voltage of cells is most conveniently obtained by measuring them with a potentio meter, but this is possible only in the laboratory.

The voltage of an unused cell is usually from r • 5o to i .6o volts. Higher voltages are sometimes found, but do not indi cate superiority of cell. Lower voltages than 1.45 volts may indicate manufacturing defects, deterioration due to age, or damage. Abnormally low values indicate probably low service capacity. Hence the open-circuit voltage test made with a volt meter is the best test available for detecting defective cells.

The open-circuit voltage, measured by a potentiometer changes by only a small amount relatively during the life of the cell. In one instance a cell under observation for 20 years still showed 1.36 volts when measured on the potentiometer, although its re sistance had increased so that a voltmeter measurement such as is described above showed only 0.215 volt.

In the short-circuit current test a deadbeat ammeter, accu rately calibrated, is used. The resistance of the lead wires and shunt of the ammeter should have a value of o•oi ohm to within 0.002 ohm. The maximum swing of the needle is taken as the short-circuit current of the cell. The lead wires are conveniently tipped with lead to make good contact and should be applied to the brass terminals of the cell. Results of tests vary with the temperature. They should be made only when the cell is at a normal room temperature ; that is, about 70° F.

Intermittent tests are made to imitate the use of cells under average conditions. The three standard intermittent tests are (I) Light intermittent tests, which represent telephone and other light services. (2) Heavy intermittent test, which represents ignition service, and (3) Flashlight test, which represents flash light service.

Besides the tests mentioned above it may be desirable to make other tests, including other physical measurements, chemical ex amination, noise in radio batteries, and the effect of tempera ture. For these no definite procedure has been established. A superficial physical examination will occasionally serve to indi cate certain defects, such as loose terminals, leaking seals and flaws in the zinc. (J. N. P.) A recent successful departure from accepted standards of "B" battery construction in the United States involves the substitution of flat cells for the usual cylindrical elements. The electrodes for these cells are made of sheet-zinc, coated on one side with a water-proof conducting plastic, the latter serving as the cathode terminal. Upon the coated side of the metal plate is stamped a mix cake consisting of a suitable mixture of manganese dioxide, carbon and electrolyte, followed by a layer of wet, pasted pulp board and a second of coated zinc, this one with its metallic sur face in contact with the pulp-board. Another mix cake is then added and the building-up process is continued until a block of the requisite number of cells has been produced. Electrical con tact between cells is obtained through the coated zinc, no separate connections being provided. The whole battery is finally placed in a box and sealed.

Lalande Ce11.

Second only to the Leclanche in commercial importance is the cell devised by Felix Lalande 0880 and Georges Chaperon. This element (see Lorenz, Z. Elektrochem., vol. iv., p. 308, 1897) consists of an amalgamated zinc anode, a caustic alkali electrolyte (usually a 15 to 25% solution of sodium hydroxide), and a copper oxide depolarizer-cathode. The copper oxide may be -either granular, in which case it is usually tamped into a perforated iron or copper container, or agglomerated and compacted into a cathode block. The use of heavily amalgamated zinc (i.o to 2.5% mercury) reduces wasteful zinc corrosion to such an extent that cell deterioration with time is very slight. The evaporation of electrolyte, or the access of carbon dioxide which is also deleterious to cell action, is obviated by pouring a layer of paraffin oil over the surface of the electrolyte. The Lalande cell has an open-circuit voltage of i.o to I.' and an operating voltage, depending on the current drain, usually rang ing from o.6 to 0.85 volt in ordinary commercial practice. De polarization is rapid and efficient, hence large currents can be drawn from the cell. During discharge zinc goes into solution, while hydrogen is liberated at the cathode where it is oxidized to water by the copper oxide ; the latter is simultaneously reduced in successive stages to metallic copper during the process. Zinc in solution appears to be largely in the form of a zincate complex Since the zinc ion concentration remains low and the voltage changes during copper oxide reduction (of the order of 0.2 volt) are small, the cell has remarkably well-sustained operat ing voltage during the period of service life. Cell action proceeds until the active electrodes are used up or, in case the electrolyte is the limiting factor, until zinc hydroxide or a hydrated sodium zincate of the type crys tallizes from the saturated solution and prevents further opera tion by forming non-conducting deposits on the active electrode surfaces. By the addition of small quantities of sulphur to the depolarizer as practised in the Columbia high voltage unit (manu factured in the United States), the operating voltage can be increased by about o.i volt.

The Lalande cell is one of the most efficient and satisfactory primary batteries known to-day for the special classes of service to which it is suited. It lends itself readily to rugged construction; it is relatively cheap to make and operate; it is very reliable in its action and has a high current output per unit of volume (about I ampere hour per 8 cc. of electrolyte) . The cell is made in units as large as Soo to i,000 ampere hour sizes. Because it requires no attention for long periods of time and because of its excellent continuous discharge and heavy duty characteristics, the Lalande cell is at present much used in railway signal operation. It can be made in dry or non-spillable form either by gelatinizing the caustic soda solution with small quantities of starch or by using such expedients as making a paste out of electrolyte and mag nesium oxide.

Air cells of the Lalande type, in which a porous carbon acces sible to air is substituted for the usual copper oxide element, are also feasible. These have an even more horizontal discharge curve than the copper oxide cell, since the potential of the cathode remains virtually unchanged during service life. The caustic soda air cell has an open circuit voltage of 1.35 to 1.45 and an operating voltage even on comparatively heavy drains above I •o volt—perhaps o•4 to o• 5 volt higher than that of a standard copper oxide cell. The carbon electrode can be used repeatedly, only zinc and electrolyte requiring renewal each time the cell is completely discharged. (G. W. H.) BIBLIOGRAPHY.-W. R. Cooper, "Primary Batteries" (London, Bibliography.-W. R. Cooper, "Primary Batteries" (London, 19or) ; Park Benjamin, "The Voltaic Cell" (New York, 1893) ; W. E. Ayrton, "Practical Electricity" (London, 1896) ; H. S. Carhart, "Primary Batteries," 1896. See also Circular of the Bureau of Standards, No. 79, "Electrical Characteristics and Testing of Dry Cells" (Washington, 1923) .