[Ill.u.s.tration: Fig. 58. Operator"s Receiver and Cord]
[Ill.u.s.tration: Fig. 59. Receiver Symbols]
Conventional Symbols. The usual diagrammatic symbols for hand and head receivers are shown in Fig. 59. They are self-explanatory. The symbol at the left in this figure, showing the general outline of the receiver, is the one most commonly used where any sort of a receiver is to be indicated in a circuit diagram, but where it becomes desirable to indicate in the diagram the actual connections with the coil or coils of the receiver, the symbol shown at the right is to be preferred, and obviously it may be modified as to number of windings and form of core as desired.
CHAPTER VII
PRIMARY CELLS
Galvani, an Italian physician, discovered, in 1786, that a current of electricity could be produced by chemical action. In 1800, Volta, a physicist, also an Italian, threw further light on Galvani"s discovery and produced what we know as the _voltaic_, or _galvanic_, cell. In honor of these two discoverers we have the words volt, galvanic, and the various words and terms derived therefrom.
Simple Voltaic Cell. A very simple voltaic cell may be made by placing two plates, one of copper and one of zinc, in a gla.s.s vessel partly filled with dilute sulphuric acid, as shown in Fig. 60. When the two plates are not connected by a wire or other conductor, experiment shows that the copper plate bears a positive charge with respect to the zinc plate, and the zinc plate bears a negative charge with respect to the copper. When the two plates are connected by a wire, a current flows from the copper to the zinc plate through the metallic path of the wire, just as is to be expected when any conductor of relatively high electrical potential is joined to one of relatively low electrical potential. Ordinarily, when one charged body is connected to another of different potential, the resulting current is of but momentary duration, due to the redistribution of the charges and consequent equalization of potential. In the case of the simple cell, however, the current is continuous, showing that some action is maintaining the charges on the two plates and therefore maintaining the difference of potential between them. The energy of this current is derived from the chemical action of the acid on the zinc. The cell is in reality a sort of a zinc-burning furnace.
In the action of the cell, when the two plates are joined by a wire, it may be noticed that the zinc plate is consumed and that bubbles of hydrogen gas are formed on the surface of the copper plate.
_Theory_. Just why or how chemical action in a voltaic cell results in the production of a negative charge on the consumed plate is not known. Modern theory has it that when an acid is diluted in water the molecules of the acid are split up or _dissociated_ into two oppositely charged atoms, or groups of atoms, one bearing a positive charge and the other a negative charge of electricity. Such charged atoms or groups of atoms are called _ions_. This separation of the molecules of a chemical compound into positively and negatively charged ions is called _dissociation_.
Thus, in the simple cell under consideration the sulphuric acid, by dissociation, splits up into hydrogen ions bearing positive charges, and SO_{4} ions bearing negative charges. The solution as a whole is neutral in potential, having an equal number of equal and opposite charges.
[Ill.u.s.tration: Fig. 60. Simple Voltaic Cell]
It is known that when a metal is being dissolved by an acid, each atom of the metal which is torn off by the solution leaves the metal as a positively charged ion. The carrying away of positive charges from a hitherto neutral body leaves that body with a negative charge. Hence the zinc, or _consumed_ plate, becomes negatively charged.
In the chemical attack of the sulphuric acid on the zinc, the positive hydrogen ions are liberated, due to the affinity of the negative SO_{4} ions for the positive zinc ions, this resulting in the formation of zinc sulphate in the solution. Now the solution itself becomes positively charged, due to the positive charges leaving the zinc plate with the zinc ions, and the free positively charged hydrogen ions liberated in the solution as just described are repelled to the copper plate, carrying their positive charges thereto. Hence the copper plate, or the _unconsumed_ plate, becomes positively charged and also coated with hydrogen bubbles.
The plates or electrodes of a voltaic cell need not consist of zinc and copper, nor need the fluid, called the _electrolyte_, be of sulphuric acid; any two dissimilar elements immersed in an electrolyte that attacks one of them more readily than the other will form a voltaic cell. In every such cell it will be found that one of the plates is consumed, and that on the other plate some element is deposited, this element being sometimes a gas and sometimes a solid.
The plate which is consumed is always the negative plate, and the one on which the element is deposited is always the positive, the current through the connecting wire always being, therefore, from the unconsumed to the consumed plate. Thus, in the simple copper-zinc cell just considered, the zinc is consumed, the element hydrogen is deposited on the copper, and the current flow through the external circuit is from the copper to the zinc.
The positive charges, leaving the zinc, or consumed, plate, and pa.s.sing through the electrolyte to the copper, or unconsumed, plate, const.i.tute in effect a current of electricity flowing within the electrolyte. The current within the cell pa.s.ses, therefore, from the zinc plate to the copper plate. The zinc is, therefore, said to be positive with respect to the copper.
_Difference of Potential._ The amount of electromotive force, that is generated between two dissimilar elements immersed in an electrolyte is different for different pairs of elements and for different electrolytes. For a given electrolyte each element bears a certain relation to another; _i.e._, they are either electro-positive or electro-negative relative to each other. In the following list a group of elements are arranged with respect to the potentials which they a.s.sume with respect to each other with dilute sulphuric acid as the electrolyte. The most electro-positive elements are at the top and the most electro-negative at the bottom.
+Sodium Lead Copper Magnesium Iron Silver Zinc Nickel Gold Cadmium Bis.m.u.th Platinum Tin Antimony -Graphite (Carbon)
Any two elements selected from this list and immersed in dilute sulphuric acid will form a voltaic cell, the amount of difference of potential, or electromotive force, depending on the distance apart in this series of the two elements chosen. The current within the cell will always flow from the one nearest the top of the list to the one nearest the bottom, _i.e._, from the most electro-positive to the most electro-negative; and, therefore, the current in the wire joining the two plates will flow from the one lowest down in the list to the one highest up.
From this series it is easy to see why zinc and copper, and also zinc and carbon, are often chosen as elements of voltaic cells. They are widely separated in the series and comparatively cheap.
This series may not be taken as correct for all electrolytes, for different electrolytes alter somewhat the order of the elements in the series. Thus, if two plates, one of iron and the other of copper, are immersed in dilute sulphuric acid, a current is set up which proceeds through the liquid from the iron to the copper; but, if the plates after being carefully washed are placed in a solution of pota.s.sium sulphide, a current is produced in the opposite direction. The copper is now the positive element.
Table II shows the electrical deportment of the princ.i.p.al metals in three different liquids. It is arranged like the preceding one, each metal being electro-positive to any one lower in the list.
TABLE II
Behavior of Metals in Different Electrolytes
+------------------+-------------------+--------------------+CAUSTIC POTASHHYDROCHLORIC ACIDPOTa.s.sIUM SULPHIDE+------------------+-------------------+--------------------++ Zinc+ Zinc+ ZincTinCadmiumCopperCadmiumTinCadmiumAntimonyLeadTinLeadIronSilverBis.m.u.thCopperAntimonyIronBis.m.u.thLeadCopperNickelBis.m.u.thNickelSilverNickel- Silver- Antimony- Iron+------------------+-------------------+--------------------+
It is important to remember that in all cells, no matter what elements or what electrolyte are used, the electrode _which is consumed_ is the one that becomes _negatively charged_ and its terminal, therefore, becomes the _negative terminal_ or _pole_, while the electrode _which is not consumed_ is the one that becomes _positively charged_, and its terminal is, therefore, the _positive terminal_ or _pole of the cell_.
However, because the current in the electrolyte flows from the _consumed_ plate to the _unconsumed_ plate, the consumed plate is called the _positive_ plate and the unconsumed, the _negative_. This is likely to become confusing, but if one remembers that the _active_ plate is the _positive_ plate, because it sends forth _positive_ ions in the electrolyte, and, therefore, itself becomes _negatively_ charged, one will have the proper basis always to determine the direction of the current flow, which is the important thing.
_Polarization._ If the simple cell already described have its terminals connected by a wire for some time, it will be found that the current rapidly weakens until it ceases to be manifest. This weakening results from two causes: first, the hydrogen gas which is liberated in the action of the cell is deposited in a layer on the copper plate, thereby covering the plate and reducing the area of contact with the liquid. This increases the internal resistance of the cell, since hydrogen is a non-conductor. Second, the plate so covered becomes in effect a hydrogen electrode, and hydrogen stands high as an electro-positive element. There is, therefore, actual reduction in the electromotive force of the cell, as well as an increase in internal resistance. This phenomenon is known as polarization, and in commercial cells means must be taken to prevent such action as far as possible.
The means by which polarization of cells is prevented or reduced in practice may be divided into three general cla.s.ses:
First--_mechanical means_. If the hydrogen bubbles be simply brushed away from the surface of the electrode the resistance and the counter polarity which they cause will be diminished. The same result may be secured if air be blown into the solution through a tube, or if the liquid be kept agitated. If the surface of the electrode be roughened or covered with points, the bubbles collect more freely at the points and are more quickly carried away to the surface of the liquid. These means are, however, hardly practical except in cells for laboratory use.
Second--_chemical means_. If a highly oxidizing substance be added to the electrolyte, it will destroy the hydrogen bubbles by combining with them while they are in a nascent state, and this will prevent the increase in internal resistance and the opposing electromotive force. Such substances are bichromate of potash, nitric acid, and chlorine, and are largely used.
Third--_electro-chemical means_. Double cells, arranged to separate the elements and liquids by means of porous part.i.tions or by gravity, may be so arranged that solid copper is liberated instead of hydrogen at a point where the current leaves the liquid, thereby entirely obviating polarization. This method also is largely used.
_Local Action._ When a simple cell stands idle, _i.e._, with its circuit open, small hydrogen bubbles may be noticed rising from the zinc electrode instead of from copper, as is the case where the circuit is closed. This is due to impurities in the zinc plate, such as particles of iron, tin, a.r.s.enic, carbon, etc. Each of these particles acts with the surrounding zinc just as might be expected of any pair of dissimilar elements opposed to each other in an electrolyte; in other words, they const.i.tute small voltaic cells.
Local currents, therefore, are generated, circulating between the two adjacent metals, and, as a result, the zinc plate and the electrolyte are needlessly wasted and the general condition of the cell is impaired. This is called _local action_.
_Amalgamated Zincs._ Local action might be prevented by the use of chemically pure zinc, but this, on account of its expense, cannot be employed commercially. Local action, however, may be overcome to a great extent by amalgamating the zinc, _i.e._, coating it with mercury. The iron particles or other impurities do not dissolve in the mercury, as does the zinc, but they float to the surface, whence the hydrogen bubbles which may form speedily carry them off, and, in other cases, the impurities fall to the bottom of the cell. As the zinc in the pasty amalgam dissolves in the acid, the film of mercury unites with fresh zinc, and so always presents a clear, bright, h.o.m.ogeneous surface to the action of the electrolyte.
The process of amalgamating the zinc may be performed by dipping it in a solution composed of
Nitric Acid 1 lb.
Muriatic Acid 2 lbs.
Mercury 8 oz.
The acids should be first mixed and then the mercury slowly added until dissolved. Clean the zinc with lye and then dip it in the solution for a second or two. Rinse in clean water and rub with a brush.
Another method of amalgamating zincs is to clean them by dipping them in dilute sulphuric acid and then in mercury, allowing the surplus to drain off.
Commercial zincs, for use in voltaic cells as now manufactured, usually have about 4 per cent of mercury added to the molten zinc before casting into the form of plates or rods.
Series and Multiple Connections. When a number of voltaic cells are joined in series, the positive pole of one being connected to the negative pole of the next one, and so on throughout the series, the _electromotive forces_ of all the cells are added, and the electromotive force of the group, therefore, becomes the sum of the electromotive forces of the component cells. The currents through all the cells in this case will be equal to that of one cell.
If the cells be joined in multiple, the positive poles all being connected by one wire and the negative poles by another, then the _currents_ of all the cells will be added while the electromotive force of the combination remains the same as that of a single cell, a.s.suming all the cells to be alike in electromotive force.
Obviously combinations of these two arrangements may be made, as by forming strings of cells connected in series, and connecting the strings in multiple or parallel.
The term battery is frequently applied to a single voltaic cell, but this term is more properly used to designate a plurality of cells joined together in series, or in multiple, or in series multiple so as to combine their actions in causing current to flow through an external circuit. We may therefore refer to a battery of so many cells. It has, however, become common, though technically improper, to refer to a single cell as a battery, so that the term battery, as indicating necessarily more than one cell, has largely lost its significance.
Cells may be of two types, primary and secondary.
Primary cells are those consisting of electrodes of dissimilar elements which, when placed in an electrolyte, become immediately ready for action.
Secondary cells, commonly called _storage cells_ and _acc.u.mulators_, consist always of two inert plates of metal, or metallic oxide, immersed in an electrolyte which is incapable of acting on either of them until a current has first been pa.s.sed through the electrolyte from one plate to the other. On the pa.s.sage of a current in this way, the decomposition of the electrolyte is effected and the composition of the plates is so changed that one of them becomes electro-positive and the other electro-negative. The cell is then, when the _charging_ current ceases, capable of acting as a voltaic cell.
This chapter is devoted to the primary cell or battery alone.