Voltaic cell

“ Ten parts of water One of the disadvantages of the voltaic cell is that it becomes polarized, that is, small bubbles of hydrogen which are liberated by the chemical action collect on the copper plate and cause the strength of the battery to fall off rapidly. ”

“ It is only by thus visualizing the activity of the atoms in a solution that we are able to provide even a thinkable hypothesis as to the development of electricity in the voltaic cell.
What puts us on the track of the explanation we are seeking is the fact that the diverse atoms are known to have different electrical properties. In our voltaic cell, for example, sodium atoms would collect at one pole and chlorine atoms at the other. Humphry Davy discovered this fact in the early days of electro-chemistry, just about a century ago. ”

“ We can also increase both volts and amperes by combining the two methods.
Fig. 9.—Cells connected in Series.
A voltaic cell gives us a considerable quantity of electricity at low pressure, the electro-motive force of a Leclanché cell being about 11⁄2 volts, and that of a Daniell cell about 1 volt. We may perhaps get some idea of the electrical conditions existing during a thunderstorm from the fact that to produce a spark one mile long through air at ordinary pressure we should require a battery of more than a thousand million Daniell cells. ”

“ This apparatus thus may be made to serve to some extent as an accumulator or storage cell, and it also serves to show that an accumulator does not store up or accumulate electricity. In a voltaic cell we have chemical energy converted into electrical energy, and here we have first electrical energy converted into chemical energy, and then the chemical energy converted back again into electrical energy. ”

“ For instance, if the electrodes of the voltaic cell are first connected by a short thick wire, and afterwards by a long thin wire, the heat developed in the cell for each grain of zinc dissolved is greater in the first case than the second, but the heat developed in the wire is greater in the second case than in the first. ”

“ A cell of a voltaic battery, or rather the surface of separation of the fluid of the cell and the zinc, is the seat of an electro-motive force which can maintain a current in opposition to the resistance of the circuit. If we adopt the usual convention in speaking of electric currents, the positive current is from the fluid through the platinum, the conducting circuit, and the zinc, back to the fluid again. ”

“ Now, if the secondary cell is separated from the primary one, we have a veritable voltaic battery, for the symmetry of the poles is upset, and one is ready to give up oxygen and the other eager to receive it. ”

“ According to this theory, then, the electrical energy in evidence in the current from the voltaic cell, is drawn from a store of potential energy in the atoms of matter composing the liquid in the cell. In practice, as is well known, the liquid used is one that affects one of the metal poles more actively than the other, insuring vigorous chemical activity. ”

“ Electricity as first Obtained by Man. Until modern times the only electricity known to us was that of the lightning flash, which man could neither hinder nor make. But in the year 1800, electricity in the form of a weak current was obtained by Volta of Italy in a very simple way; and even now our various electric batteries and cells are but a modification of that used by Volta and called a voltaic cell. A strip of [307] copper and a strip of zinc are placed in a glass containing dilute sulphuric acid, a solution composed of oxygen, hydrogen, sulphur, and water. ”

“ Where more than a small amount of current is required, the dynamo and the storage battery have generally taken their place as sources of electric current.
The advantages of voltaic cells as electric generators are: (a) they are inexpensive, (b) they are easily taken from place to place, (c) they may be ready for instant use.
The most desirable voltaic cell would be one having the [Pg 275] following qualities: (a) High electromotive force, (b) no polarization or local action, (c) very low internal resistance, (d) small expense, both as to first cost and upkeep.
Fig. ”

“ Electrolysis. It has been seen that in the voltaic cell electricity is generated by chemical action. Sulphuric acid acts upon zinc and dissolves it in the cell, hydrogen is produced, etc. When this process is reversed, that is, when the electric current is passed through some solutions, they are decomposed, or broken up into their constituents. This process is called electrolysis, and the compound decomposed is the electrolyte. (See "Study," § 369, etc., with experiments.) ”

“ The electro-motive force of a voltaic cell is undoubtedly due to the intrinsic superficial manifestation of energy. When two dissimilar metals are placed in connection with each other, either directly or through the medium of a conducting liquid, the chemical action of the liquid brings new surfaces of the metals constantly in contact. ”

“ The burning of zinc in a voltaic cell is indeed such a fire, and most of the attempts to transform the latent energy of coal into electrical energy efficiently have been attempts to construct a voltaic cell which will burn coal. ”

“ But as the wire has to be charged, enough current to
move the relay may not reach the end for some seconds.
Capacity of Polarization of a Voltaic Cell.
The relative resistance to polarization of a voltaic cell, measured by
the quantity of electricity it can supply before polarization. A
counter-electromotive force may be developed, or the acid or other
solution may become exhausted. ”

“ Suppose two fairly clean pieces of metal in light contact—say two pieces of brass or of iron—connected to a single voltaic cell; a film of what may be called oxide intervenes between the surfaces so that only an insignificant current is allowed to pass, because a volt or two is insufficient to break down the insulating film, except perhaps at one or two atoms. [14] If the film is not permitted to conduct at all, it is not very sensitive; the most sensitive condition is attained when an infinitesimal current passes, strong enough just to show on a moderate galvanometer. ”

“ Volta soon found that it was not necessary to have his pairs of metals in actual metallic contact, and that better results were got by placing them in a vessel filled with dilute acid. Fig. 7 is a diagram of a simple voltaic cell of30 this kind, and it shows the direction of the current when the zinc and the copper are connected by the wire. In order to get some idea of the reason why a current flows we must understand the meaning of electric potential. If water is poured into a vessel, a certain water pressure is produced. ”

“ The researches of M. Gaston Planté on the polarization of voltameters led to his invention of the secondary cell, composed of two strips of lead immersed in acidulated water. These cells accumulate, and, so to speak, store up the electricity passed into them from some outside generator. When the two electrodes are connected with any source of electricity the surfaces of the two strips of lead undergo certain modifications. Thus, the positive pole retains oxygen and becomes covered with a thin coating of peroxide of lead, while the negative pole becomes reduced to a clean metallic state. ”

“ If such an electrolytic cell has a continuous electromotive force applied to it tending to force a current through the electrolyte from the platinum wire to the lead plate, we can distinguish three stages in its operation, which are determined by the electromotive force and the inductance in the circuit. First, if the electromotive force is below sixteen or twenty volts, then ordinary and silent electrolysis of the liquid proceeds, bubbles of oxygen being liberated from the platinum wire and hydrogen set free against the lead plate. ”

“ When the new compounds are allowed to go back to their original condition by joining the electrodes of the charged cell a current is produced. In other words, an electric current [157] produces chemical changes in the cell by electrolysis, and these new compounds have an E. M. F. of polarization because they are constantly willing and anxious to get back to their old state. The plates are lead and are usually coated with compounds of lead. Hydrogen and oxygen are given out at the electrodes. The current from a dynamo is used to charge secondary batteries. ”

“ Lead zinc cells are similar to the preceding type, but differ by having zinc for the negative electrode, and zinc sulphate for the electrolyte. The voltage of these cells is slightly higher than that of the ordinary cell, and their capacity per unit of total weight is high, but they are apt to lose their charge on open circuit, besides they possess most of the disadvantages of the Plante cells. ”

“ The action of the Voltaic current upon the brain is exceedingly powerful, and the greatest care must be exercised when it is applied to any part of the head or face. Not more than two or three cells should be commenced with. Its effect should be noted, and any increase should be made gradually. With a very moderate current giddiness is produced, and upon the sudden application of a [80] sufficiently powerful one the patient falls down as if struck by a blow. ”

“ It will often happen that the electrodes used may be of such a nature as, with the fluid in which they are immersed, to produce an electric current, either according with or opposing that of the voltaic arrangement used, and in this way, or by direct chemical action, may sadly disturb the results. Still, in the midst of all these confusing effects, the electric current, which actually passes in any direction through the body suffering decomposition, will produce its own definite electrolytic action. ”

“ What is positive and what negative electricity?
No difference in electricity in itself. When a body has more than its natural amount of electricity, it is said to be charged positively; when it has less than its natural amount it is negatively charged.
What is a Cell; what a Battery?
If a piece of zinc and copper joined by a wire be dipped in a liquid—generally weak sulphuric acid—which will act chemically on the metals, a current is produced. Such an arrangement is called a couple, or cell. If many cells are connected, then it is called a battery. ”

“ The Samson cell has a large zinc plate bent in the form of a cylinder, the carbon-manganese combination standing in the centre of it.
The dry cell also has a large zinc, the internal resistance being thus much lowered, the current output is increased. This is by reason of Ohm’s law, which teaches that to increase the current flow, either the voltage of the battery must be increased, or the resistance decreased.
But increased current means lessened life; there is only just so much energy in a cell mainly dependent on the quantity of chemicals. ”

“ Under such circumstances it is especially imperative, for there is great belief, even with educated persons who ought to know better, that the benefit derived from electrization is in exact proportion to the pain given by it, and a little personal experience is a very wholesome corrective to such a notion. Graduate your dose, then, by the number of cells employed checked by testing its effect, and do this upon each application, for there is no certain means of securing that the strength of either a Voltaic or a Faradaic current shall not have varied from day to day. ”

“ Open circuit cells polarize quickly, because the depolarizer (§ 280) is slow in destroying the hydrogen.
When a strong current is needed for a considerable time, such as for telegraph lines, motors, etc., a closed circuit cell is necessary. The depolarization must be rapid and constant. The bichromate (§ 289) and the Daniell cell (§ 290) are very common forms of closed circuit cells. (See, also, Storage Cells.)
287. The Leclanché Cell is an open circuit cell in which carbon and zinc are the plates. The carbon is surrounded with dioxide of manganese, a depolarizer ”

“ In order to procure a reference to _chemical action_, the wires were now retained immersed in the acid to the depth of five eighths of an inch, and the needle, when stationary, observed; it stood, as nearly as the unassisted eye could decide, at 5-1/3 division. Hence a permanent deflection to that extent might be considered as indicating a constant voltaic current, which in eight beats of my watch (369.) could supply as much electricity as the electrical battery charged by thirty turns of the machine. ”

“ If the wires are connected to an electric bell, the bell will ring, or, instead, the current may be used to run a small motor. If the cell is made of two zinc plates or two copper plates, the bell will not ring, because no electricity will be produced. In order to produce a current, the electrodes must be made of two different materials upon which the acid acts differently. Current may be obtained from a cell made with a zinc and carbon plate or from one with zinc and iron. ”

“ For instance, if a number of electrolytic cells of dilute sulphuric acid with platinum electrodes are arranged in series, and if a small electromotive force, such as that of one Daniell's cell, be made to act on the circuit, the electromotive force will produce a current of exceedingly short duration, for after a very short time the electromotive force arising from the polarization of the cell will balance that of the Daniell's cell. ”