Hawkins Electrical Guide, Number One Part 9

=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.

[Ill.u.s.tration: 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 gla.s.s 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.

[Ill.u.s.tration: FIG. 55.--Wheelock cell; the elements are carbon and zinc.

To set up, place the grid in the bottom of the jar and fill its two troughs each about half full of mercury. Place the porous cell in position on the grid so that it sits perfectly upright, resting in the recess of the latter. The zincs stand with lower ends resting in mercury in the troughs of the grid. Into the porous cell, to a height of _only two-thirds_ full, pour solution consisting of equal parts water and sulphuric acid, by measure. Add to this 1/2 pound nitrate soda, 1 ounce chromic acid. This solution may be made up in the above proportion and kept in covered receptacle in any desired quant.i.ty, ready for use. In the outer jar for 6 8 size, 2-1/2 pints of water, and 1/2 gill sulphuric acid, 1 part sulphuric acid to 20 parts water, or as much sulphuric acid as it will take without boiling. When a charge becomes exhausted it may be renewed by adding sulphuric acid and salts in the proportions given above, after drawing out with syringe enough of the old solutions to make room for the additions, but the best action is obtained with entirely new solutions. Zincs must be kept thoroughly amalgamated by keeping a good supply of mercury in the troughs.]

When a current is produced by a Daniell cell:

1. Copper is deposited on the copper plate; 2. Copper sulphate is consumed; 3. The sulphuric acid remains unchanged in quant.i.ty; 4. Zinc sulphate is formed; 5. Zinc is consumed.

If, however, the copper sulphate solution be too weak, the water is decomposed instead of the copper sulphate, and hydrogen is deposited on the copper plate. This deposit of hydrogen lowers the voltage, hence care should be taken to maintain an adequate supply of copper sulphate.

The voltage of a Daniell cell varies from about 1.07 volt to 1.14 volt, according to the density of the copper sulphate solution and the amount of zinc sulphate present in the dilute sulphuric acid.

="Dry" Cells.=--It is often necessary to use cells in places where there is considerable jarring or motion, as for automobile or marine ignition.

The ordinary cell is not well adapted to this service on account of the liability of spilling the electrolyte, hence, the introduction of the so-called dry cell.

A dry cell is composed of two elements, usually zinc and carbon, and a liquid electrolyte. A zinc cup closed at the bottom and open at the top forms the negative electrode; this is lined with several layers of blotting paper or other absorbing material.

The positive electrode consists of a carbon rod placed in the center of the cup; the s.p.a.ce between is filled with carbon--ground c.o.ke and dioxide of manganese mixed with an absorbent material. This filling is moistened with a liquid, generally sal-ammoniac. The top of the cell is closed with pitch to prevent leakage and evaporation. A binding post for holding the wire connections is attached to each electrode and each cell is placed in a paper box to protect the zincs of adjacent cells from coming into contact with each other when finally connected together to form a battery.

=Points Relating to Dry Cells.=--The following instructions on the care and operation of dry cells should be carefully noted and followed to get the best results:

[Ill.u.s.tration: FIGS. 56 and 57.--Round and rectangular types of the so-called "dry" cell.]

1. In renewing dry cells (or any other kind of cell), a greater number should never be put in series than was originally required to do the work, because the additional cells increase the voltage beyond that required, which causes more current than is necessary to flow through the coil. This increased current flow shortens the life of the battery.

2. In connecting dry cells in places where there is vibration, heavy copper wire should not be used, because vibration will cause it to break.

3. Water should not be allowed to come in contact with the paper covers of the cells because they form the insulation, hence, when moist, current will leak across from one cell to another, resulting in running down the battery.

4. Dry cells will deteriorate when not in use, making it necessary to renew them about every sixty days. The reason dry cells deteriorate is because the moisture evaporates. Freezing, exposure to heat, and vibration which loosens the sealing, causes the evaporation.

5. Weak cells can be strengthened somewhat by removing the paper jacket, punching the metal cup full of small holes, and then placing in a weak solution of sal-ammoniac, allowing the cells to absorb all they will take up. This is only to be recommended in cases of emergency when they are hard to get.

[Ill.u.s.tration: FIGS. 58 to 63.--Various zincs; fig. 58 Fuller; fig. 59 Daniell; fig. 60 Leclanche square; fig. 61 Leclanche round; fig. 62 Sampson; fig. 63, bottle.]

6. The average voltage of a dry cell when new is one and one-half volts, while the amperage ranges from about twenty-five to fifty amperes according to size.

7. A dry cell when fresh should show from =20= to =25= amperes when tested; the date of manufacture should also be noted as fresh cells are most efficient.

8. Dry cells should be tested with an ammeter, care being taken to do it quickly as the ammeter being of a very low resistance short circuits the cell. A volt meter is not used in testing because, while the cells are not giving out current, their voltage remains practically the same, and a cell that is very weak will show nearly full voltage. When no ammeter is at hand, the battery current may be tested by disconnecting the end of one of the terminal wires and snapping it across the binding post of the other terminal; the intensity of the spark produced will indicate the condition of the battery.

=Points Relating to the Care of Cells.=--To get the best results from primary cells, they should receive proper attention and be maintained in good condition. The instructions here given should be carefully followed.

[Ill.u.s.tration: FIGS. 64 to 66: Various carbons; fig. 61 Cylindrical form; fig. 65 Calland star; fig. 66, wheel.]

=Cleanliness.=--In the care of batteries, cleanliness is essential in order to secure best results. Zincs and coppers should be thoroughly cleaned every time a cell is taken out of use. The zinc, after being thoroughly cleaned, should be rubbed with a little mercury. This prevents local action. Porous cups should be soaked in clean water four or five hours and then wiped dry.

The terminals of each cell should be thoroughly cleansed and sc.r.a.ped bright so as to get good contact of the connecting wires and thus avoid extra resistance in the circuit.

=Separating the Elements.=--Obviously the positive and negative elements of a cell must not be in contact within the exciting fluid; they should be separated by a s.p.a.ce of 3/8 to 1/2 inch. In the case of cells without porous cups, periodic attention must be given to ensure this condition being maintained.

[Ill.u.s.tration: FIGS. 67 to 69.--Various zincs: fig. 67 Crowfoot; fig. 68 Lockwood; fig. 69 fire alarm.]

=Creeping.=--As evaporation of the electrolyte takes place in a cell, it increases in strength, and crystals are left on the sides of the jar previously wetted by the solution, the action being very marked when the solution is a saturated one. The s.p.a.ce between these crystals and the side of the jar acts as a number of capillary tubes, and draws up more liquid, which itself evaporates and deposits crystals above the former ones. So that finally the film of crystals pa.s.ses over the edge of the jar and forms on the outside, thus making a kind of syphon which draws off the liquid. This action may, to a great extent, be prevented by warming the edges of the gla.s.s, or stoneware, jars, and of the porous pots, before the cells are made up, and dipping them while warm into some paraffin wax melted in warm oil, a precaution that should always be carried out when a dense solution of zinc sulphate is employed in the cell.

=Amalgamated Zinc.=--To "amalgamate" a piece of zinc, dip it into dilute sulphuric acid to clean its surface, then rub a little mercury over it by means of a piece of rag tied on to the end of a stick, and lastly, leave the zinc standing for a short time in a dish to catch the surplus mercury as it drains off.

[Ill.u.s.tration: FIGS. 70 and 71.--Two forms of copper element: fig. 70, regular form for crowfoot cell; fig. 71, signal pan bottom copper.]

The action of the amalgamated zinc is not well understood; by some it is considered that amalgamating the zinc prevents local currents by the amalgam _mechanically_ covering up the impurities on the surface of the zinc and preventing their coming into contact with the liquid. By others it is thought that amalgamating the zinc protects it from local action by causing a film of hydrogen gas to adhere to it. This theory is based on the fact that while no action takes place when amalgamated zinc is placed in dilute sulphuric acid at ordinary atmospheric pressure, the creation of a vacuum above the liquid causes a rapid evolution of hydrogen, which, however, stops on the readmission of the air.

Amalgamating a zinc causes it to act as a somewhat more positive substance than before, therefore the voltage of a cell containing amalgamated zinc is slightly higher than that of a cell constructed with unamalgamated zinc.

[Ill.u.s.tration: FIG. 72.--Diagram of a series battery connection: four cells are shown connected by this method. If the cell voltage be one and one-half volts, the pressure between the (+) and (-) terminals of the _battery_ is equal to the product of the voltage of a single cell multiplied by the number of cells. For four cells it is equal to six volts.]

The addition of a very small amount of zinc to mercury causes the mercury to act as if it were zinc alone, arising perhaps from the amalgam having the effect of bringing the zinc to the surface.

=Battery Connections.=--There are three methods of connecting cells to form a battery; they may be connected:

1. In series; 2. In parallel; 3. In series multiple.

A series connection consists in joining the positive pole of one cell to the negative pole of the other, as shown in fig. 72; this adds the voltage of each cell.

Thus, connecting in series four cells of one and one-half volts each will give a total of six volts.

Fig. 73 ill.u.s.trates a parallel or multiple connection; this is made by connecting the positive terminal of one cell with the positive terminal of another cell and the negative terminal of the first cell with the negative terminal of the second cell.

[Ill.u.s.tration: FIG. 73.--Diagram of a multiple or parallel connection.

When connected in this manner the voltage of the battery is the same as that of a single cell, but the current is equal to the amperage of a single cell multiplied by the number of cells. Thus with 1-1/2 volt 15 ampere dry cells, the combination or battery connected as shown would give 4 15 = 60 amperes at a pressure of 1-1/2 volts.]

[Ill.u.s.tration: FIG. 74.--Diagram of a series multiple connection. Two sets of cells are connected in series and the two batteries thus formed, connected in parallel. The pressure equals the voltage of one cell, multiplied by the number of cells in one battery, and the amperage, that of one cell multiplied by the number of batteries. This form of connection is objectionable unless all the cells be of equal strength. If old cells be placed on one side and new cells on the other, current will flow (as in fig. 75) from the stronger through the weaker until the pressure of all the cells thus becomes equal. This process therefore wastes some of the energy of the strong cells.]

_A paralleled or multiple connection adds the amperage of each cell; that is, the amperage of the battery will equal the sum of the amperage of each cell._

For instance, four cells of twenty-five amperes each would give a total of one hundred amperes when connected in parallel.

A series multiple connection, fig. 74, consists of two series sets of cells connected in parallel. In series multiple connections the voltage of each set of cells or battery must be equal, or the batteries will be weakened, hence each battery of a series multiple connection should contain the same number of cells.