Volume I Part 8 (1/2)
[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 POTASH
HYDROCHLORIC ACID
POTa.s.sIUM SULPHIDE
+------------------+-------------------+--------------------+
+ Zinc
+ Zinc
+ Zinc
Tin
Cadmium
Copper
Cadmium
Tin
Cadmium
Antimony
Lead
Tin
Lead
Iron