[Ill.u.s.tration: 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 pa.s.s 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 cla.s.s, 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 cla.s.s.

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 gla.s.s, 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 gla.s.s 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 gla.s.s 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 1-1/2 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 s.p.a.ce 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 pota.s.sium, 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 gla.s.s 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 pa.s.ses through the porous cup and combines with the oxygen in the bichromate of pota.s.sium. 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 gla.s.s jar, as in fig. 46, and fill gla.s.s jar to within about three inches of the top with a mixture of three parts of electropoion fluid to two parts of water.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 b.u.t.tons.

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.

[Ill.u.s.tration: 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 gla.s.s 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, 1-1/4 pints of boiling water, and 3 ounces of sulphuric acid.]

The cell consists of a gla.s.s 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 bra.s.s 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.

[Ill.u.s.tration: 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 pa.s.ses through the porous part.i.tion, 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 pota.s.sium is frequently subst.i.tuted 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 pa.s.ses 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 gla.s.s jar (an ordinary gla.s.s 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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.]