Hawkins Electrical Guide, Number One Part 7

=Ques. If the electrodes be connected externally what happens?=

Ans. If the plates be connected by a wire outside the electrolyte, the tendency which dissimilar electrical charges have to neutralize one another causes a flow of negative electricity through the wire from zinc to copper, and a positive flow in the opposite direction. The "static"

charge being thus disposed of, a fresh charge is given to the plates by the action of the acid, which commences to dissolve the zinc. As long as the wire connects the copper and zinc plates, the acid will continue its action on the zinc until either acid or zinc is exhausted.

The reader may ask: how can there be a positive flow when both plates are negatively electrified?

An a.n.a.logy is the best way to make this point clear: Imagine two equal vessels, from each of which the air has been partially exhausted, but from one (A) 10 times as much air has been taken as from the other (B). Connect A and B by a tube. Now, although both vessels have less than the atmospheric pressure, that is, both have "negative" pressures, yet a current of air will flow from B to A until the pressures in each are equalized; that is, until both have equal "negative charges" of air.

There is a second important effect of the acid solution or electrolyte in a cell. If pure sulphuric acid were used, the first action or production of an electrical charge on the zinc plate would be the same, but when the plates were joined by the wire the current would soon cease. The reason for this lies in the fact that the sulphate of zinc, which is the compound produced by the acid plus the zinc, being insoluble in pure undiluted sulphuric acid, remains on the surface of the zinc plate. The coating of sulphate of zinc thus formed also operates as a protective agent, and no further electrical charge can be induced until it is removed. The addition of water to the acid has the effect of allowing the sulphate of zinc to dissolve, and the zinc plate is left free for further action.

=Ques. What governs the rate of current flow of a primary cell?=

Ans. The size of the elements and their proximity.

=Effects of Polarization.=--The film of hydrogen bubbles affects the strength of the current of the cell in two ways:

1. It weakens the current by the increased _resistance_ which it offers to the flow, for bubbles of gas are bad conductors;

2. It weakens the current by setting up an opposing _electromotive force_.

Hydrogen is almost as oxidizable a substance as zinc, especially when freshly deposited (in the "nascent" state), and is electro-positive; hence, the hydrogen itself produces a difference of potential, which would tend to start a current in the opposite direction to the true zinc-to-copper current. It is therefore an important matter to abolish this polarization, otherwise the currents furnished by batteries would not be constant.

=Methods of Depolarizing.=--One of the chief aims in the arrangement of the numerous cells which have been devised is to avoid polarization. The following are the methods usually employed:

1. Chemical methods;

_a_. Oxidation of the hydrogen by pota.s.sium b.i.+.c.hromate and by nitric acid.

_b_. Subst.i.tution of the hydrogen by some other substance which does not give a counter electromotive force of polarization; for instance, in the Daniell cell by replacement of the copper in copper sulphate by the hydrogen, the copper being deposited on the positive pole.

2. Electro-chemical means;

It is possible by employing double cells, to secure such action that some solid metal, such as copper, shall be liberated instead of hydrogen bubbles, at the point where the current leaves the liquid. This electro-chemical exchange obviates polarization.

3. Mechanical methods.

_a_. Agitation of the liquid or of the positive electrode, in order to prevent the acc.u.mulation of hydrogen thereon.

_b_. Corrugating or roughing the positive electrode, as in the Smee cell. This causes the hydrogen gas to form in large bubbles which rise to the surface more rapidly than the small bubbles which form on a smooth electrode.

In the simplest form of cell, as zinc, copper, and dilute sulphuric acid, no attempt has been made to prevent the evil of polarization, hence, it will quickly polarize when the current is closed for any length of time, and may be cla.s.sified as an open circuit cell.

When polarization is remedied by chemical means, the chemical added is one that has a strong affinity for hydrogen and will combine with it, thus preventing the covering of the negative plate with the hydrogen gas.

[Ill.u.s.tration: FIGS. 42 and 43.--Carbon cell and carbon cylinder. Carbon possesses a natural power to prevent a limited amount of polarization by absorbing the hydrogen gas coming from the zinc rod; hence it is used in various shapes for open circuit cells, which gives rise to as many different names, such as _Samson_, _Hercules_, _Law_, _National_, _Standard_, etc. In all these types of cell, sal-ammoniac and zinc are used, and by corrugating the carbon, fluting it, or making concentric cylinders, special merits are obtained in each case. The carbon element is usually made in the form of a porous cup, filled with oxide of manganese to prevent polarization, and then sealed. The zinc rod is inserted through a porcelain insulator. About 4 to 6 ounces of sal-ammoniac are generally used for cells of ordinary size. The salt is placed in the jar, water poured in until it is about two-thirds full, and then stirred till all the salt is dissolved. When the carbon cylinder is inserted, the solution should be within 1-1/2 inches of the top of the jar. The electromotive force is from 1.0 to 1.4 volts for the different forms of carbon cell.]

=Ques. What is a depolarizer?=

Ans. A substance employed in some types of cell to combine with the hydrogen which would otherwise be set free at the positive electrode and cause polarization.

The chemical used for this purpose may be either in a _solid_ or _liquid_ form, which gives rise to several types of cell, such as cells with a single fluid, containing both the acid and the depolarizer, cells with a single exciting fluid and a solid depolarizer, and cells with two separate fluids.

In the two fluid cell, the zinc is immersed in the liquid (frequently dilute sulphuric acid) to be decomposed by the action upon it, and the negative plate is surrounded by the liquid depolarizer, which will be decomposed by the hydrogen gas it arrests, thereby preventing polarization.

In _open circuit cells_ polarization does not have much opportunity to occur, since the circuit is closed for such a short period of time; hence, these cells are always ready to deliver a strong current when used intermittently.

In _closed circuit cells_ polarization is prevented by chemical action, so that the current will be constant and steady till the energy of the chemicals is expended.

=Ques. What is a depolarizer bag?=

Ans. A cylinder of hemp or other fabric used in place of a porous pot in some forms of Leclanche cell, and also as a support for the depolarizing ma.s.s in some forms of dry cell where the electrolyte is of a thin gelatinous nature.

=Volta's Contact Law.=--When metals differing from each other are brought into contact, different results are obtained, both as to the kind of electrification as well as the difference of potentials.

Volta found that iron, when in contact with zinc, becomes negatively electrified; the same takes place, but somewhat weaker, when iron is touched with lead or tin. When, however, iron is touched by copper or silver, it becomes positively electrified. Volta, Seebeck, Pfaff, and others have investigated the behavior of many metals and alloys when in contact with each other.

The following lists are so arranged that those metals first in each list become positively electrified when touched by any taking rank after them:

CONTACT SERIES OF METALS

_According to Volta_. _According to Pfaff._ + zinc + zinc lead cadmium tin tin iron lead copper tungsten silver iron gold bis.m.u.th graphite antimony - manganese ore copper silver gold uranium tellurium platinum - palladium

Volta laid down a law regarding the position of the metals in his table which may be stated as follows:

_The difference of potential between any two metals is equal to the sum of the differences of potentials of all the intermediate members of the series._

Hence, it is immaterial for the total effect whether the first and the last are brought into contact directly, or whether the contact is brought about by means of all or any of the intermediate metals.

Volta's law further a.s.serts that when any number of metals are brought into contact with each other, but so that the chain closes with the metal with which it was begun, the total difference must be zero.

=Laws of Chemical Action in the Cell.=--There are two simple laws of chemical action in the cell:

1. _The amount of chemical action in a cell is proportional to the quant.i.ty of electricity that pa.s.ses through it._

One coulomb of electricity in pa.s.sing through the cell liberates .000010352 of a gramme of hydrogen, and causes .00063344 of a gramme of zinc to dissolve in the acid.

2. _The amount of chemical action is equal in each cell of a battery connected in series._

=Requirements of a Good Cell.--=The several conditions which should be fulfilled by a good cell are as follows:

1. Its electromotive force should be high and constant; 2. Its internal resistance should be small; 3. It should be perfectly quiescent when the circuit is open; 4. It should give a constant current, and therefore must be free from polarization, and not liable to rapid exhaustion; 5. It should be easily cared for, and if possible, should not emit corrosive fumes; 6. It should be cheap and of durable materials.

=Single and Two Fluid Cells.=--The distinction between a single and a two fluid cell has already been given. The single fluid cell of Volta with its zinc and copper plates represents the simplest form of primary cell.

In the _two fluid cell_, the positive (zinc) plate is immersed in the exciting liquid (usually dilute sulphuric acid) and is decomposed by the action upon it, while the negative plate is placed in the liquid depolarizer which is decomposed by the hydrogen arrested by it, thus preventing polarization.

In some forms of cell, the two liquids are separated by a porous part.i.tion of unglazed earthenware, which, while it prevents the liquids mixing except very slowly, does not prevent the pa.s.sage of hydrogen and electricity.