A circular water circuit with water-column pressure indicators.
Fig. 38.—Hydrostatic analogy of fall of potential in an electrical circuit.
Voltmeter is connected to any two points in circuit.
Fig. 39.—Showing method of connecting voltmeter to find potential difference between any two points as m and n on an electrical circuit.

Strength of Current.—It is important that the reader have a clear conception of this term, which is so often used. The exact definition of the strength of a current is as follows:

The strength of a current is the quantity of electricity which flows past any point of the circuit in one second.

Example.—If, during 10 seconds, 25 coulombs of electricity flow through a circuit, then the average strength of the current during that time is 21⁄2 coulombs per second, or 21⁄2 amperes.

Voltage Drop in an Electric Circuit.—A difference of potential exists between any two points on a conductor through which a current is flowing on account of the resistance offered to the current by the conductor.

For instance, in the electrical circuit shown in fig. 39, the potential at the point a is higher than that at m, that at m higher than that at n, etc., just as in the water circuit, shown in fig. 38, the hydrostatic pressure at a is greater than that at m′, that at m′ greater than that at n′, etc. The fall in the water pressure between m′ and n′ (fig. 38) is measured by the water head n’s.

In order to measure the fall in electrical potential between m and n, (fig. 39), the terminals of a volt meter are placed in contact with these points as shown. Its reading will give the difference of potential between m and n, in volts, provided that its own current carrying capacity is so small that it does not appreciably lower the potential difference between the points m and n by being touched across them; that is, provided the current which flows through it is negligible in comparison with that which flows through the conductor which already joins the points m and n.

CHAPTER IV

PRIMARY CELLS

The word “battery” is a much abused word, being often used incorrectly for “cell,” as in fig. 40. Hence, careful distinction should be made between the two terms.

A battery consists of two or more cells joined together so as to form a single unit.

There are numerous forms of primary cell; they may be classified as follows:

1. According to the service for which they are designed;

2. According to the chemical features.

With respect to the first method cells are classified as:

1. Open circuit cells;

Used for intermittent work, where the cell is in service for short periods of time, such as in electric bells, signaling work, and electric gas lighting. If kept in continuous service for any length of time the cell soon polarizes or “runs down,” but will recuperate after remaining on open circuit for some little time.

2. Closed circuit cells.

This type of cell is adapted to furnishing current continuously, as in telegraphy, etc.

With respect to the second method, cells are classified as:

1. One fluid;
2. Two fluid;

Ques. Describe a primary cell.

Ans. A primary cell consists of a vessel containing a liquid in which two dissimilar metal plates are immersed.

In one fluid cells both metal plates are immersed in the same solution. In two fluid cells each metal plate is immersed in a separate solution, one of which is contained in a porous cup which is immersed in the other liquid.

Ques. What name is given to the metal plates?

Ans. They are called elements.

Ques. What is the fluid called?

Ans. The electrolyte or exciting fluid.

The term “electropoion” is a trade name for the electrolyte employed in the Fuller cell.

Action of a Primary Cell.—The fundamental fact on which the electro-chemical generation of current depends is, that if a plate of metal be placed in a liquid there is a difference of electrical condition produced between them of such sort that the metal either takes a lower or higher electrical potential than the liquid, according to the nature of the metal and the liquid. If two different metals be placed in one electrolytic liquid, then there is a difference of state produced between them, so that, if joined by wire outside the liquid, a current of electricity will traverse the wire. This current proceeds in the liquid from the metal which is most acted upon chemically to that which is least acted upon.

Referring to fig. 41, the construction and action of a simple primary cell may be briefly described as follows:

Place in a glass jar some water having a little sulphuric or other acid added to it. Place in it separately two clean strips, one of zinc, Z, and one of copper, C. This cell is capable of supplying a continuous flow of electricity through a wire whose ends are brought into connection with the two strips. When the current flows, the zinc strip is observed to waste away, its consumption in fact furnishing the energy or electromotive force required to drive the current through the cell and the connecting wire. The cell may therefore be regarded as a kind of chemical furnace in which the fuel is the zinc.

Fig. 40.—Simple primary cell. It consists of two dissimilar metal plates (such as copper and zinc which are called the elements), immersed in the electrolyte or exciting fluid contained in the glass jar.

Ques. How are the positive and negative elements of a primary cell distinguished?

Ans. The plate attacked by the electrolyte is the negative element, and the one unattacked the positive element.

Chemical Changes; Polarization.—The chemical changes which take place in a simple cell, consisting of zinc and copper elements in an electrolyte of dilute sulphuric acid, may be briefly described as follows: When the two elements are connected and the current commences to flow, the sulphuric acid acts on the surface of the zinc plate and forms sulphate of zinc. The formation of this new substance necessitates the liberation of some of the hydrogen contained in the sulphuric acid, and it will be found that bubbles of free hydrogen gas speedily appear on the surface of the negative element, that is, on the copper plate.

While the zinc is being dissolved to form zinc sulphate, hydrogen gas is liberated from the sulphuric acid.

Fig. 41.—Simple primary cell with circuit closed, showing direction of the current.

Some bubbles of the gas rise to the surface of the electrolyte and so escape into the air, but much of it clings to the surface of the copper element which thus gradually becomes covered with a thin film of hydrogen.

Partly on account of the decreased area of copper plate in contact with the electrolyte, and partly because the hydrogen tends to produce a current in the opposite direction, the useful electrical output becomes considerably diminished and the cell is said to be polarized. This state of affairs may be rectified by stirring up the electrolyte, or by shaking the cell, so as to assist the hydrogen bubbles to detach themselves from the surface of the copper plate and make their way to the atmosphere through the electrolyte. This, however, is only a temporary remedy, as the polarized condition will soon be reached again, and a further agitation of the cell will be necessary. Hence, a simple cell of this kind is not desirable for practical work, and it must be modified to adapt it to constant use.

When the sulphuric acid in a cell acts in the zinc element and produces sulphate of zinc, a certain amount of work is done which is manifested partly in the form of useful electric energy, and partly as heat which warms the electrolyte and which is thereby lost for all practical purposes.

Ques. If the zinc and copper electrodes of a simple cell be not connected externally what changes take place within the cell?

Ans. The zinc plate immediately becomes strongly charged with negative electricity, and the copper plate weakly so. As long as the plates remain unconnected, and the zinc is pure, no further action takes place.

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 analogy 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 potassium bichromate and by nitric acid.

b. Substitution 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 accumulation 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 classified 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.

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 11⁄2 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 mass 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 bismuth
 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 asserts 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 quantity of electricity that passes through it.

One coulomb of electricity in passing 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 partition of unglazed earthenware, which, while it prevents the liquids mixing except very slowly, does not prevent the passage of hydrogen and electricity.

Glass jar with cylindrical electrodes.
Figs. 44 and 45.—Leclanche cell and porous cup. This very common form of cell is an example of the single fluid type, with a solid depolarizer surrounding the negative element; the latter is generally carbon, the positive element being zinc. The liquid used is a strong solution of ammonium chloride, commonly known as sal-ammoniac, and which resembles table salt. In the porous cup type of cell, a carbon slab is placed in the porous cup, and is surrounded by a mixture of small pieces of carbon and manganese dioxide, the top being covered by means of pitch, leaving one or two small holes for air and gas to pass through. The depolarizer will take care of a limited amount of the hydrogen produced when the cell is on closed circuit, but if the circuit be closed for any length of time polarization occurs. The cell is thus of the open circuit class, and will furnish a good current where it is required only intermittently. Zinc is dissolved only when the cell is being used. This type of cell, or its modification, is used for gas lighting and bell work. The cell requires very little attention. Water must be added as the solution evaporates, and the zinc rod replenished when necessary. The electromotive force is about 1.48 volts and the internal resistance about 4 ohms.

Complete depolarization is usually obtained also in single fluid cells, having in addition a depolarizing solid body, such as oxide of manganese, oxide of copper, or peroxide of lead, in contact with the carbon pole. Such cells really do not belong to the single fluid cells, and are considered in the two fluid class.

A few examples of single and double fluid primary cells will now be described.

The Leclanche Cell.—This cell was invented by Leclanche, a French electrician, and was the first cell in which sal-ammoniac was used. This form of cell, as shown in fig. 45, is in general use for electric bells, its great recommendation being that, once charged, it retains its power without attention for considerable time.

Two jars are employed in its construction; the outer one is of glass, contains a zinc rod, and is charged with a solution of ammonium chloride, called sal-ammoniac.

The inner jar is of porous earthenware, containing a carbon plate, and is filled with a mixture of manganese peroxide and broken gas carbon. When the carbon plate and the zinc rod are connected, a steady current of electricity is set up, the chemical action which takes place being as follows: the zinc becomes oxidized by the oxygen from the manganese peroxide, and is subsequently converted into zinc chloride by the action of the sal-ammoniac.

After the battery has been in continuous use for some hours, the manganese becomes exhausted of oxygen, and the force of the electrical current is greatly diminished; but if the battery be allowed to rest for a short time, the manganese obtains a fresh supply of oxygen from the atmosphere, and is again fit for use.

After about 18 months work, the glass cell will probably require recharging with sal-ammoniac, and the zinc rod may also need renewing; but should the porous cell get out of order, it is better to get a new one than to attempt to recharge it.

The directions for setting up a Leclanche cell are as follows:

1. Place in the glass jar six ounces of sal-ammoniac, and pour in water until the jar is one-third full, then stir thoroughly.
2. Place the porous cup in the solution, and if necessary add water until it rises to within 11⁄2 inches of the top of the porous cup.
3. Put the zinc rod in place and set the cell away (not connected up), for about 12 hours, so as to allow the liquid to thoroughly soak into the porous cup. This will lower the level of the liquid to about one-third the height of the jar. The cell will then be ready for use. As the level of the liquid is lowered by evaporation, it should be maintained at the stated height by adding water.

The Leclanche cell is adapted to open circuit work, being extensively used for ringing electric bells.

The objections to the Leclanche cell are:

1. Rapid polarization;
2. High internal resistance due to porous pot;
3. Restricted space for electrolyte causing rapid lowering of level of liquid by evaporation;
4. Eating away of the zinc rod at the surface of the liquid, rendering the rod useless before the lower part is consumed.

Fuller Bichromate Cell.—In the bichromate cells or the chromic acid cells, bichromate of soda, or bichromate of potassium, is used for the depolarizer, water and sulphuric acid being added for attacking the zinc.

The Fuller cell is of the two fluid type. A pyramidal block of zinc at the end of a metallic rod covered with gutta-percha is placed in the bottom of a porous cup containing an ounce of mercury. The cup is then filled with a very dilute solution of sulphuric acid or water and placed in a jar of glass or earthenware containing the bichromate solution and the carbon plate. The diffusion of the acid through the porous cup is sufficiently rapid to attack the zinc, which being well amalgamated, prevents local action; while the hydrogen passes through the porous cup and combines with the oxygen in the bichromate of potassium. This type of cell has an electromotive force of 2.14 volts, and is suited to open circuit, or semi-closed circuit work. The directions for setting up a Fuller cell are as follows:

1. To make the “electropoion” fluid, mix together one gallon of sulphuric acid and three gallons of water, and in a separate vessel, dissolve six pounds of bichromate of potash in two gallons of boiling water; then thoroughly mix together the two solutions.
2. Immerse the zinc in a solution of dilute sulphuric acid, and then in a bath of mercury, and rub it with a brush or cloth so as to reach all parts of the surface.
3. Pour into the porous cell one ounce (a tablespoonful) of mercury, and fill the porous cell with water up to within two inches of the top.
4. Place the porous cell and the carbon plate in the glass jar, as in fig. 46, and fill glass jar to within about three inches of the top with a mixture of three parts of electropoion fluid to two parts of water.
Figs. 46 and 47.—The telephone standard and compound forms of the Fuller cell. The type shown in fig. 46 is especially adapted to long distance telephoning, and that shown in fig. 47 to incandescent lamps, motors, nickel and other electroplating. The Fuller cell is a double fluid variety and has the advantage over the Grenet type, in that the zinc is always kept well amalgamated and does not require removal from the solution. The Fuller cell is suitable for open and semi-closed circuit work; its electromotive force is about 2.14 volts.

5. The zinc should be lifted out occasionally and the sulphate washed off.
6. The supply of mercury in the porous cell should be maintained, so as to have the zinc always well amalgamated.
7. To renew, clean all deposits from carbon plate and zinc, and set up with fresh solution.

The Edison Cell.—This is a single fluid cell with a solid depolarizer, as shown in fig. 48, and is well adapted for use on closed circuits.

Fig. 48.—Edison cell, type R R. The electrolyte used is caustic soda, the positive element zinc, and the negative element copper oxide. The Edison cell is suitable for large stationary gas engine ignition, railroad crossing signals, electroplating, fire alarms, telephone circuits, etc.

The positive element is zinc, and the negative element black oxide of copper. The exciting fluid is a solution of caustic potash. The black oxide of copper plates are suspended from the cover of the jar by a light framework of copper, one end of which forms the positive pole of the battery. A zinc plate is suspended on each side of the copper oxide element and kept from coming in contact with the latter by means of vulcanite buttons.

When the cell is in action, the water is decomposed, and the oxygen thus liberated combines with the zinc and forms oxide of zinc, which combines with the potash to form a double salt of zinc and potash. The last combination dissolves as rapidly as it is formed. The hydrogen liberated by the decomposition of the water reduces the copper oxide to pure metallic copper. It is highly important that the copper oxide plates be completely submerged in the solution of caustic potash, and that heavy paraffin oil be poured on top of the solution to the depth of about 1⁄4 of an inch to exclude the air. If oil be not used, the formation of creeping salts will reduce the life of the battery fully two-thirds. The battery has a low electromotive force, about 0.7 of a volt, but as the internal resistance is also very low, quite a large current can be drawn from the cell.

The Bunsen Cell, shown in figs. 49 and 50, is a two fluid cell constructed with zinc and carbon electrodes. The negative plate is carbon, the positive plate amalgamated zinc. The excitant is a dilute solution of sulphuric acid. The top part of the carbon is sometimes impregnated with paraffin (to keep the acid from creeping up).

The force of the Bunsen cell increases after setting up for about an hour, and the full effect is not attained until the acid soaks through the porous cell. Carbons are not affected and last any length of time. The zinc is slowly consumed through the mercury coating.

Grenet Bichromate Cell.—In this cell, as shown in figs. 49 and 50, the positive element is zinc and the negative element carbon. The electrolyte is a solution of bichromate of potash in a mixture of sulphuric acid and water.

Figs. 49 and 50.—American and French forms of Grenet cell. The elements are zinc and carbon. In the Grenet cell, a zinc plate is suspended by a rod between two carbon plates, so that it does not touch them, and when the cell is not in use the zinc is withdrawn from the solution by raising and fastening the rod by means of a set screw, as the acid attacks the zinc when the cell is on open circuit. This cell has an electromotive force of over 2 volts at first, and gives a strong current for a short time, but the liquid soon becomes exhausted, as will be noted by the change in the color of the solution from an orange to a dark red, and must be replenished. The zinc should be kept well amalgamated and out of the solution except when in use. It is a good type of cell for experimental work. To make the electrolyte take 3 ounces of finely powdered bichromate of potash and 1 pint of boiling water; stir with a glass rod and after it is cool, add slowly, stirring all the time, 3 ounces of sulphuric acid. The electrolyte may also be prepared as follows: take 4 ounces of bichromate of soda, 11⁄4 pints of boiling water, and 3 ounces of sulphuric acid.

The cell consists of a glass bottle containing the electrolyte and fitted with a lid from which the elements are supported. There is a zinc plate in the center and a carbon plate on each side. The two carbon plates are connected to the same terminal, thus forming a large negative surface, and the zinc plate to a terminal on the top of the brass rod to which it is attached. This rod slides through a hole in the lid so that the zinc plate can be lifted out of the electrolyte when the cell is not at work, thus preventing wasteful consumption of zinc and of the electrolyte. Bichromate cells give a strong current, the electromotive force of a single cell being 2 volts.

Fig. 51.—The Bunsen cell. This is a two fluid cell and has a bar of carbon immersed in strong nitric acid contained in a porous cup. This cup is then placed in another vessel, containing dilute sulphuric acid, and immersed in the same liquid, is a hollow cylindrical plate of zinc, which nearly surrounds the porous cup. The hydrogen, starting at the zinc, traverses by composition and recomposition, the sulphuric acid; it then passes through the porous partition, and enters into chemical action with the nitric acid, so that none of it reaches the carbon. Water is produced by this action, which in time dilutes the acid, and orange colored poisonous fumes of nitric oxide rise from the battery. If the nitric acid first be saturated with nitrate of ammonia, the acid will last longer and the fumes be prevented. Strong sulphuric acid cannot be used in any battery; one part of sulphuric acid is generally added to 12 parts by weight, or 20 by volume, of water. Grove used a strip of platinum instead of carbon in his cell. A solution of bichromate of potassium is frequently substituted for the nitric acid in the porous cup, thereby avoiding disagreeable fumes. Bunsen’s and Grove’s cells produce powerful and constant currents, and are well adapted for experiments, but they require frequent attention, and are expensive, so that they are little used for work of long duration. The electromotive force of these cells is from 1.75 to 9.51 volts.

Daniell Cell.—This is one of the best known and most widely used forms of primary cell. It is a double fluid cell, composed of an inner porous vessel containing an electrolyte of either dilute sulphuric acid or dilute zinc sulphate solution, and an outer vessel containing a saturated solution of copper sulphate.

A zinc rod is placed in the inner electrolyte, and a thin plate of sheet copper in the outer electrolyte. Sometimes this arrangement of the elements is modified, the outer vessel being made of copper and serving as the copper plate. This would then contain the copper sulphate solution, while the zinc sulphate and the zinc rod would be contained in the porous pot as before.

The chemical reactions which take place in a Daniell cell are as follows:

The zinc dissolves in the dilute acid, thus producing zinc sulphate, and liberating hydrogen gas. The free hydrogen passes through the walls of the porous pot, but when it reaches the copper sulphate solution it displaces some of the copper therefrom, and combines with this solution, forming sulphuric acid. The copper, which is thus set free, is deposited on the surface of the copper plate. In this way polarization is avoided, and a practically constant current is obtained.

When the zinc sulphate solution is employed in place of dilute acid, a similar series of chemical reactions occur, except that the zinc is liberated instead of hydrogen.

Daniell cells are used especially for electroplating, electrotyping and telegraphic work. The electromotive force of a single cell is 1.079 volts.

Directions for Making a Daniell Cell.—The simple Daniell cell shown in fig. 52 may be easily made as follows: The outer vessel A, consists of a glass jar (an ordinary glass jam jar will do) containing a solution of sulphuric acid (1 part in 12 to 20 parts of water), and a zinc rod B.

Inside the jar is placed a porous pot C containing a strip of thin sheet copper D, and a saturated solution of sulphate of copper (also called “blue stone” and “blue vitrol”).

The zinc is preferably of the Leclanche form, which will be found to be cleaner, more durable, and cheaper than a zinc sheet. The porous pot should be dipped in melted paraffin wax, both top and bottom, to prevent the solution mingling too freely and “creeping.” A few crystals of copper sulphate are placed in the pot as shown.

Fig. 52.—Simple Daniell cell for closed circuit work. To maintain a constant current for an indefinite time, it is only necessary to maintain the supply of copper crystals and zinc. The cell as shown in the figure is easily made by following the direction given in the accompanying text.

In mixing the sulphuric acid and water, the acid should be added to the water—never the reverse. Zinc sulphate is sometimes used instead, as it reduces the wasteful consumption of the zinc, but it should be pure.

With care the cell will last for weeks. When it weakens or “runs down,” an addition of sulphuric acid to the outer jar and a few more crystals placed in the porous pot will put the cell in good condition.

The two electrodes are segregated vertically.
Fig. 53.—Daniell gravity cell, “crowfoot” pattern. This is a two fluid cell in which gravity instead of a porous cup is depended upon to keep the liquids separate. The two solutions consist of copper sulphate and dilute sulphuric acid, the elements being made of zinc and copper.

Gravity Cells.—In a two liquid cell, instead of employing a porous cell to keep the two liquids separate, it is possible, where one of the liquids is heavier than the other, to arrange that the heavier liquid shall form a stratum at the bottom of the cell, the lighter floating upon it. Such arrangements are called gravity cells; but the separation is never perfect, the heavy liquid slowly diffusing upwards.

Daniell Gravity Cell.—In this cell, shown in fig. 53, the same elements are used as in the ordinary Daniell cell, but the porous pot is dispensed with, the two solutions being separated by the action of gravity as explained in the preceding paragraph.

Fig. 54.—Partz acid gravity cell. In this form of cell, the electrolyte which surrounds the zinc is either magnesium sulphate or common salt. The depolarizer is a bichromate solution which surrounds the perforated carbon plate located in the bottom of the jar. A vertical carbon rod fits snugly into the tapered hole in the carbon plate, and extends through the cover forming the positive pole. The depolarizer, being heavier than the electrolyte, remains at the bottom of the jar, and the two liquids are thus kept separate. This depolarizer is placed on the market in the form of crystals, known as sulpho-chromic salt, made by the action of sulphuric acid upon chromic acid. When dissolved, its action is similar to that of the chromic acid solution. After the cell has been set up with everything else in place, the crystals are introduced into the solution, near the bottom of the jar, through the vertical glass tube shown, and slowly dissolve and diffuse over the surface of the carbon plate. When the cell current weakens a few tablespoonfuls of the salt introduced through the tube will restore the current to its normal value. The cell should remain undisturbed to prevent the solution from mixing. Its electromotive force is from 1.9 to 2 volts, and the 6 in. × 8 in. size has an internal resistance of about .5 ohm. Since the depolarizer is quite effective, the cell may be used on open or closed circuit work.

The copper sulphate solution, being the heavier of the two, rests at the bottom of the battery jar, while the dilute sulphuric acid remains at the top. To suit this arrangement the copper and zinc elements are located as shown, the copper elements being at the bottom, and the zinc element, shaped like a crow’s foot (hence the name “crowfoot cell”) is suspended at the top.

The absence of the porous pot decreases the internal resistance, but the electromotive force is the same as in the ordinary type of Daniell cell.