CHAPTER IV.VARIOUS VOLTAIC CELLS.

Fig. 35.

44. Early Experiments.In 1786 Galvani, an Italian physician, made experiments to study the effect of static electricity upon the nervous excitability of animals, and especially upon the frog. He found that electric machines were not necessary to produce muscular contractions or kicks of the frog's legs, and that they could be produced when two different metals, Fig. 35, like iron and copper, for example, were placed in proper contact with a nerve and a muscle and then made to touch each other. Galvani first thought that the frog generated the electricity instead of the metals.

Volta proved that the electricity was caused by the contact of the metals. He used the condensing electroscope as one means of proving that two dissimilar metals become charged differently when in contact. Volta alsocarried out his belief by constructing what is called aVoltaic Pile. He thought that by making several pairs of metals so arranged that all the little currents would help each other, a strong current could be generated. Fig. 36 shows apile, it being made by placing a pair of zinc and copper discs in contact with one another, then laying on the copper disc a piece of flannel soaked in brine, then on top of this another pair, etc., etc. By connecting the first zinc and the last copper, quite a little current was produced. This was a start from which has been built our present knowledge of electricity. Strictly speaking, electricity is not generated by combinations of metals or by cells; they really keep up a difference of potential, as will be seen.

Fig. 36.

Fig. 37.Fig. 38.

Fig. 37.

Fig. 38.

45. The Simple Cell.It has been stated that two different kinds of electrifications may be produced by friction; one positive, the other negative. Either can be produced, at will, by using proper materials. Fig. 37 shows a section of asimple cell; Fig. 38 shows another view. Cu is a piece of copper, and Zn a piece of zinc. When they are placed indilute sulphuric acid, it can be shown by delicate apparatus that they become charged differently, because the acid acts differently upon the plates. They become charged by chemical action, and not by friction. The zinc is gradually dissolved, and it is this chemical burning of the zinc that furnishes energy for the electric current in the simple cell. The electrification, or charge, on the plates tends to flow from the place of higher to the place of lower potential, just as water tends to flow down hill. If a wire be joined to the two metals, a constant current of electricity will flow through it, because the acid continues to act upon the plates. The simple cell is asingle-fluidcell, as but one liquid is used in its construction.

45a. Plates and Poles.The metal strips used in voltaic cells are calledplatesorelements. The one most acted upon by the acid is called the positive (+) plate. In the simple cell the zinc is the + plate, and the copper the negative (-) plate. The end of a wire attached to the - plate is called the + pole, or electrode. Fig. 37 shows the negative (-) electrode as the end of the wire attached to the + plate.

46. Direction of Current.In the cell the current passes from the zinc to the copper; that is, from the positive to the negative plate, where bubbles of hydrogen gas are deposited. In the wire connecting the plates, the current passes from the copper to the zinc plate. In most cells, carbon takes the place of copper. (See "Study," § 268.)

47. Local Currents; Amalgamation.Ordinary zinc contains impurities such as carbon, iron, etc., andwhen the acid comes in contact with these, they form with the zinc a small cell. This tends to eat away the zinc without producing useful currents. The little currents in the cell from this cause are calledlocal currents. (See "Study," Exp. 111, § 273.) This is largely overcome by coating the zinc with mercury. This process is calledamalgamation. It makes the zinc act like pure zinc, which is not acted upon by dilute sulphuric acid when the current does not pass. (See "Study," § 257, 274.)

48. Polarization of Cells.Bubbles of hydrogen gas are formed when zinc is dissolved by an acid. In the ordinary simple cell these bubbles collect on the copper plate, and not on the zinc plate, as might be expected. The hydrogen is not a conductor of electricity, so this film of gas holds the current back. The hydrogen acts like a metal and sets up a current that opposes the zinc to the copper current. Several methods are employed to get rid of the hydrogen. (See "Study," § 278, 279, 280.)

49. Single-Fluid and Two-Fluid Cells.The simple cell (§ 45) is a single-fluid cell. The liquid is called theelectrolyte, and this must act upon one of the plates; that is, chemical action must take place in order to produce a current. The simple cell polarizes rapidly, so something must be used with the dilute sulphuric acid to destroy the hydrogen bubbles. This is done in thebichromate of potash cell.

In order to get complete depolarization—that is, to keep the carbon plate almost perfectly free from hydrogen, it is necessary to usetwo-fluid cells, or those to which some solid depolarizer is added to the one fluid.

50. Open and Closed Circuit Cells.If we consider a voltaic cell, the wires attached to it, and perhaps some instrument through which the current passes, we have anelectric circuit. When the current passes, the circuit isclosed, but when the wire is cut, or in any way disconnected so that the current can not pass, the circuit isopenorbroken. (See "Study," § 266.)

Open Circuit Cellsare those which can give momentary currents at intervals, such as are needed for bells, telephones, etc. These must have plenty of time to rest, as they polarize when the circuit is closed for a long time. TheLeclanchéanddrycells are the most common open circuit cells.

Closed Circuit Cells.For telegraph lines, motors, etc.,where a current is needed for some time, the cell must be of such a nature that it will not polarize quickly; it must give a strong and constant current. Thebichromateandgravity cellsare examples of this variety. (See "Study," § 286.)

Fig. 39.

51. Bichromate of Potash Cellsare very useful for general laboratory work. They are especially useful for operating induction coils, small motors, small incandescent lamps, for heating platinum wires, etc. These cells have an E.M.F. of about 2 volts. Dilute sulphuric acid is used as the exciting fluid, and in this is dissolved the bichromate of potash which keeps the hydrogen bubbles from the carbon plate. (See "Apparatus Book," § 26.) Zinc and carbon are used for the plates, the + pole being the wire attached to the carbon.

Fig. 39 shows one form of bichromate cell. It furnishes a large quantity of current, and as the zinc can be raised from the fluid, it may be kept charged ready for use for many months, and can be set in action any time when required by lowering the zinc into the liquid. Two of these cells will burn a one candle-power miniature incandescent lamp several hours. The carbon is indestructible.

Note.For various forms of home-made cells, see "Apparatus Book," Chapter I., and for battery fluids see Chapter II.

52. The Grenet Cell.Fig. 40 is another form of bichromate cell. The carbon plates are left in the fluid constantly. The zinc plate should be raised when the cell is not in use, to keep it from being uselessly dissolved.

Fig. 40.Fig. 41.

Fig. 40.

Fig. 41.

53. Plunge Batteries.Two or more cells are often arranged so that their elements can be quickly lowered into the acid solution. Such a combination, Fig. 41, is called aplunge battery. The binding-posts are so arranged that currents of different strengths can be taken from the combination. The two binding-posts on the right of the battery will give the current of one cell; the two binding-posts on the left of the battery will give the current of two cells, and the two end binding-posts will give the current of all three cells. When not in use the elements must always be hung on the hooks and kept out of the solution.

54. Large Plunge Batteries. Fig. 42, are arranged with a winch and a bar above the cells; these afford a ready and convenient means of lifting or lowering the elements and avoiding waste. In the battery shown, Fig. 42, the zincs are 4×6 inches; the carbons have the same dimensions, but there are two carbon plates to each zinc, thus giving double the carbon surface.

Fig. 42.

55. The Fuller Cell, Fig. 43, is another type of bichromate cell, used largely for long-distance telephone service, for telephone exchange and switch service, for running small motors, etc. It consists of a glass jar, a carbon plate, with proper connections, a clay porous cup, containing the zinc, which is made in the form of a cone. A little mercury is placed in the porous cup to keep the zinc well amalgamated. Either bichromate of potash or bichromate of soda can be used as a depolarizer.

Fig. 43.

Fig. 44.

56. The Gravity Cell, sometimes called thebluestoneorcrowfootcell, is used largely for telegraph, police, and fire-alarm signal service, laboratory and experimental work, or whenever a closed circuit cell is required. The E.M.F. is about one volt. This is a modified form of the Daniell cell. Fig. 44 shows a home-made gravity cell.

A copper plate is placed at the bottom of the glass jar, and upon this rests a solution of copper sulphate (bluestone). The zinc plate is supported about four inches above the copper, and is surrounded by a solution of zinc sulphate which floats upon the top of the blue solution. An insulated wire reaches from the copper to the top of the cell and forms the positive pole. (See "Apparatus Book," § 11 to 15, for home-made gravity cell, its regulation, etc. For experiments with two-fluid Daniell cell, see "Study," Exp. 113, § 281 to 286.)

Fig. 45.

56a. Bunsen Cells,Fig. 45, are used for motors, small incandescent lamps, etc. A carbon rod is inclosed in a porous cup, on the outside of which is a cylinder of zinc that stands in dilute sulphuric acid, the carbon being in nitric acid.

57. The Leclanché Cellis an open circuit cell. Sal ammoniac is used as the exciting fluid, carbon and zinc being used for plates. Manganese dioxide is used as the depolarizer; this surrounds the carbon plate, the two being either packed together in a porous cup or held together in the form of cakes. The porous cup, or pressed cake, stands in the exciting fluid. The E. M. F. is about 1.5 volts.

Fig. 46.

Fig. 47.

drawingsFig. 48.

Fig. 48.

Fig. 46 shows a form with porous cup. The binding-post at the top of the carbon plate forms the + electrode, the current leaving the cell at this point.

Fig. 49.

The Gonda Prism Cell(Fig. 47), is a form of Leclanché in which the depolarizer is in the form of a cake.

Fig. 50.

58. Dry Cellsare open circuit cells, and can be carried about, although they are moist inside. The + pole is the end of the carbon plate. Zinc is used as the outside case and + plate. Fig. 48 shows the ordinary forms.

Fig. 49 shows a number of dry cells arranged in a box with switch in front, so that the current can be regulated at will.

59. The Edison-Lelande Cells, Fig. 50, are made in several sizes and types. Zinc and copper oxide, which is pressed into plates, form the elements. The exciting fluid consists of a 25 per cent. solution of caustic potash in water. They are designed for both open and closed circuit work.

60. Electrical Connections.In experimental work, as well as in the everyday work of the electrician, electrical connections must constantly be made. One wire must be joined to another, just for a moment, perhaps, or one piece of apparatus must be put in an electric circuit with other apparatus, or the current must be turned on or off from motors, lamps, etc. In order to conveniently and quickly make such connections, apparatus called push-buttons, switches and binding-posts are used.

Fig. 51.Fig. 52.

Fig. 51.

Fig. 52.

61. Push-Buttons.The simple act of pressing your finger upon a movable button, or knob, may ring a bell a mile away, or do some other equally wonderful thing. Fig. 51 shows a simple push-button, somewhat like a simple key in construction. If we cut a wire, through which a current is passing, then join one of the free ends to the screw A and the other end to screw C, we shall be able to let the current pass at any instant by pressing the spring B firmly upon A.

Push-buttons are made in all sorts of shapes and sizes. Fig. 52 gives an idea of the general internal construction.The current enters A by one wire, and leaves by another wire as soon as the button is pushed and B is forced down to A. The bottom of the little button rests upon the top of B.

Fig. 53 shows aTable Clamp-Pushfor use on dining-tables, card-tables, chairs, desks, and other movable furniture. Fig. 54 shows a combination of push-button, speaking-tube, and letter-box used in city apartment houses. Fig. 55 shows anIndicating Push. The buzzer indicates, by the sound, whether the call has been heard; that is, the person called answers back.

Fig. 53.Fig. 54.

Fig. 53.

Fig. 54.

Modificationsof ordinary push-buttons are used for floor push-buttons, on doors, windows, etc., for burglar-alarms, for turning off or on lights, etc., etc. (See"Apparatus Book," Chapter III., for home-made push-buttons.)

draiwngFig. 55.

Fig. 55.

62. Switcheshave a movable bar or plug of metal, moving on a pivot, to make or break a circuit, or transfer a current from one conductor to another.

Fig. 56 shows asingle point switch. The current entering the pivoted arm can go no farther when the switch is open, as shown. To close the circuit, the arm is pushed over until it presses down upon the contact-point. For neatness, both wires are joined to the under side of the switch or to binding-posts.

Fig. 56.

Fig. 57 shows aknife switch. Copper blades are pressed down between copper spring clips to close the circuit. The handle is made of insulating material.

Pole-changing switches, Fig. 58, are used for changing or reversing the poles of batteries, etc.

Fig. 59 shows a home-made switch, usefulin connection with resistance coils. By joining the ends of the coils A, B, C, D, with the contact-points 1, 2, 3, etc., more or less resistance can be easily thrown in by simply swinging the lever E around to the left or right. If E be turned to 1, the current will be obliged to pass through all the coils A, B, etc., before it can pass out at Y. If E be moved to 3, coils A and B will be cut out of the circuit, thus decreasing the resistance to the current on its way from X to Y. Current regulators are made upon this principle. (See "Apparatus Book," Chapter IV., for home-made switches.)

Fig. 57.

Fig. 58.

Fig. 59.

Switchboardsare made containing from two or three to hundreds ofswitches, and are used in telegraph and telephone work, in electric light stations, etc., etc. (See Chapter on Central Stations.) Fig. 60 shows a switch used for incandescent lighting currents.

Fig. 60.

Fig. 61.

63. Binding-Postsare used to make connections between two pieces of apparatus, between two or more wires, between a wire and any apparatus, etc., etc. They allow the wires to be quickly fastened or unfastened to the apparatus. A large part of the apparatus shown in this book has binding-posts attached. Fig. 61 shows a few of the common forms used. (See "Apparatus Book," Chapter V., for home-made binding-posts.)

64. Electrical Units.In order to measure electricity for experimental or commercial purposes, standards or units are just as necessary as the inch or foot for measuring distances.

65. Potential; Electromotive Force.If water in a tall tank be allowed to squirt from two holes, one near the bottom, the other near the top, it is evident that the force of the water that comes from the hole at the bottom will be the greater. The pressure at the bottom is greater than that near the top, because the "head" is greater.

When a spark of static electricity jumps a long distance, we say that the charge has a highpotential; that is, it has a high electrical pressure. Potential, for electricity, means the same as pressure, for water. The greater the potential, orelectromotive force(E.M.F.) of a cell, the greater its power to push a current through wires. (See "Study," § 296 to 305, with experiments.)

66. Unit of E.M.F.; the Volt.—In speaking of water, we say that its pressure is so many pounds to the square inch, or that it has a fall, or head, of so many feet. We speak of a current as having so many volts; for example, we say that a wire is carrying a 110-volt current. The volt is the unit of E.M.F. An ordinary gravity cell has an E.M.F. of about one volt. This name was given in honor of Volta.

67. Measurement of Electromotive Force.There are several ways by which the E.M.F. of a cell, for example, can be measured. It is usually measuredrelatively, by comparison with the E. M. F. of some standard cell. (See "Study," Exp. 140, for measuring the E. M. F. of a cell by comparison with the two-fluid cell.)

Fig. 62.

Voltmetersare instruments by means of which E. M. F. can be read on a printed scale. They are a variety of galvanometer, and are made with coils of such high resistance, compared with the resistance of a cell or dynamo, that the E. M. F. can be read direct. The reason for this will be seen by referring to Ohm's law ("Study," § 356); the resistance is so great that the strength of the current depends entirely upon the E. M. F.

Fig. 63.

Voltmeters measure electrical pressure just as steam gauges measure the pressure of steam. Fig. 62 shows one form of voltmeter. Fig. 63 shows a voltmeterwith illuminated dial. An electrical bulb behind the instrument furnishes light so that the readings can be easily taken.

68. Electrical Resistance.Did you ever ride down hill on a hand-sled? How easily the sled glides over the snow! What happens, though, when you strike a bare place, or a place where some evil-minded person has sprinkled ashes? Does the sled pass easily over bare ground or ashes? Snow offers very littleresistanceto the sled, while ashes offer a great resistance.

Fig. 64.

All substances do not allow the electric current to pass through them with the same ease. Even the liquid in a cell tends to hold the current back and offersinternal resistance. The various wires and instruments connected to a cell offerexternal resistance. (See "Study," Chapter XVIII., for experiments, etc.)

69. Unit of Resistance.The Ohmis the name given to the unit of resistance. About 9 ft. 9 in. of No. 30 copper wire, or 39 feet 1 in. of No. 24 copper wire, will make a fairly accurate ohm.

Resistance coils, having carefully measured resistances, are made for standards. (See "Apparatus Book," Chapter XVII., for home-made resistance coils.) Fig. 64 shows a commercial form of a standard resistance coil. The coil is inclosed in a case and has large wires leadingfrom its ends for connections. Fig. 65 gives an idea of the way in which coils are wound and used with plugs to build upresistance boxes, Fig. 66.

70. Laws of Resistance.1. The resistance of a wire is directly proportional to its length, provided its cross-section, material, etc., are uniform.

2. The resistance of a wire is inversely proportional to its area of cross-section; or, in other words, inversely proportional to the square of its diameter, other things being equal.

Fig. 65.

3. The resistance of a wire depends upon its material, as well as upon its length, size, etc.

4. The resistance of a wire increases as its temperature rises. (See "Study," Chapters XVIII. and XIX., for experiments on resistance, its measurement, etc.)

Fig. 66.

71. Current Strength.The strength of a current at the end of a circuit depends not only upon theelectrical pressure, or E. M. F., which drives the current, but also upon theresistancewhich has to be overcome.The greater the resistance the weaker the current at the end of its journey.

72. Unit of Current Strength; The Ampere.A current having an E. M. F. ofone volt, pushing its way through a resistance ofone ohm, would have a unit of strength, calledone ampere. This current, one ampere strong, would deposit, under proper conditions, .0003277 gramme of copper inone secondfrom a solution of copper sulphate.

73. Measurement of Current Strength.A magnetic needle is deflected when a current passes around it, as in instruments like the galvanometer. Thegalvanoscopemerely indicates the presence of a current.Galvanometersmeasure the strength of a current, and they are made in many forms, depending upon the nature and strength of the currents to be measured. Galvanometers are standardized, or calibrated, by special measurements, or by comparison with some standard instrument, so that when the deflection is a certain number of degrees, the current passing through it is known to be of a certain strength.

Fig. 67.

Fig. 67 shows anastatic galvanometer. Fig. 68 shows atangent galvanometer, in which the strength of the currentis proportional to the tangent of the angle of deflection. Fig. 69 shows aD'Arsonval galvanometer, in which a coil of wire is suspended between the poles of a permanent horseshoe magnet. The lines of force are concentrated by the iron core of the coil. The two thin suspending wires convey the current to the coil. A ray of light is reflected from the small mirror and acts as a pointer as in other forms of reflecting galvanometers.

Fig. 68.

74. The Ammeter, Fig. 70, is a form of galvanometer in which the strength of a current, in amperes, can be read. In these the strength of current is proportional to the angular deflections. The coils are made with a small resistance, so that the current will not be greatly reduced in strength in passing through them.

Fig. 69.

75. Voltametersmeasure the strength of a current by chemical means, the quantity of metal deposited or gas generated being proportional to the time that the current flows and to its strength. In thewater voltameter, Fig. 71, the hydrogen and oxygen produced in agiven time are measured. (See "Study," Chapter XXI.)

Fig. 70.

Thecopper voltametermeasures the amount of copper deposited in a given time by the current. Fig. 72 shows one form. The copper cathode is weighed before and after the current flows. The weight of copper deposited and the time taken are used to calculate the current strength.

Fig. 71.

76. Unit of Quantity;The Coulombis the quantity of electricity given, inone second, by a current having astrength of one ampere. Time is an important element in considering the work a current can do.

Fig. 72.

77. Electrical Horse-power;The Wattis the unit of electrical power. A current having the strength of one ampere, and an E. M. F. of one volt has a unit of power. 746 watts make one electrical horse-power. Watts = amperes × volts. Fig. 73 shows a direct reading wattmeter based on the international volt and ampere. They save taking simultaneous ammeter and voltmeter readings, which are otherwise necessary to get the product of volts and amperes, and are also used on alternating current measurements.

Fig. 73.

There are also forms of wattmeters, Fig. 74, in which the watts are read from dials like those on an ordinary gas-meter, the records being permanent.

Fig. 75 shows a voltmeter V, and ammeter A, so placed in the circuit that readings can be taken. D represents a dynamo. A is placed so that the whole current passes through it, while V is placed between the main wires to measure the difference in potential. The product of the two readings in volts and amperes gives the number of watts.

Fig. 74.

78. Chemical Metersalso measure the quantity of current that is used; for example, one may be placed in the cellar to measure the quantity of current used to light the house.

Fig. 75.

Fig. 76 shows a chemical meter, a part of the current passing through a jar containing zinc plates and a solution of zinc sulphate. Metallic zinc is dissolved from one plate and deposited upon the other. The increase in weight shows the amount of chemical action which is proportional to the ampere hours. Knowing the relation between the quantity of current that can pass through the solution to that which can pass through the meter byanother conductor, a calculation can be made which will give the current used. A lamp is so arranged that it automatically lights before the meter gets to the freezing-point; this warms it up to the proper temperature, at which point the light goes out again.

Fig. 76.

79. 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 calledelectrolysis, and the compound decomposed is theelectrolyte. (See "Study," § 369, etc., with experiments.)

Fig. 77.

Fig. 77 shows how water can be decomposed into its two constituents, hydrogen and oxygen, there being twice as much hydrogen formed as oxygen.

Fig. 78 shows a glass jar in which are placed two metalstrips, A and C, these being connected with two cells. In this jar may be placed various conducting solutions to be tested. If, for example, we use a solution of copper sulphate, its chemical formula being CuSO4, the current will break it up into Cu (copper) and SO4. The Cu will be deposited upon C as the current passes from A to C through the solution. A is called theanode, and C thecathode.

Fig. 78.

Fig. 79 shows another form of jar used to study the decomposition of solutions by the electric current.

Fig 79.

80. Ions.When a solution is decomposed into parts by a current, the parts are called theIons. When copper sulphate (Cu SO4) is used, the ions are Cu, which is a metal, and SO4, called an acid radical. When silver nitrate (Ag NO3) is used, Ag and NO3are the ions. The metal part of the compound goes to the cathode.

81. Electricity and Chemical Action.We have just seen, Chapter VII., that the electric current has the power to decompose certain compounds when they are in solution. By choosing the right solutions, then, we shall be able to get copper, silver, and other metals set free by electrolysis.

82. Electroplatingconsists in coating substances with metal with the aid of the electric current. If we wish to electroplate a piece of metal with copper, for example, we can use the arrangement shown in Fig. 78, in which C is the cathode plate to be covered, and A is a copper plate. The two are in a solution of copper sulphate, and, as explained in § 79, the solution will be decomposed. Copper will be deposited upon C, and the SO4part of the solution will go to the anode A, which it will attack and gradually dissolve. The SO4, acting upon the copper anode, makes CuSO4again, and this keeps the solution at a uniform strength. The amount of copper dissolved from the copper anode equals, nearly, the amount deposited upon the cathode. The metal is carried in the direction of the current.

If we wish to plate something with silver or gold, it will be necessary to use a solution of silver or gold for the electrolyte, a plate of metallic silver or gold being used for the anode, as the case may be.

Great care is used in cleaning substances to be plated, all dirt and grease being carefully removed.

Fig. 80 shows a plating bath in which several articles can be plated at the same time by hanging them upon a metal bar which really forms a part of the cathode. If, for example, we wish to plate knives, spoons, etc., with silver, they would be hung from the bar shown, each being a part of the cathode. The vat would contain a solution of silver, and from the other bar would be hung a silver plate having a surface about equal to that of the combined knives, etc.

Fig. 80.

Most metals are coated with copper before they are plated with silver or gold. When plating is done on a large scale, a current from a dynamo is used. For experimental purposes a Gravity cell will do very well. (See "Study," § 374 to 380 with experiments.)

83. Electrotyping.It was observed by De La Rue in 1836 that in the Daniell cell an even coating of copper was deposited upon the copper plate. From this was developed the process of electrotyping, which consists inmaking a copy in metal of a wood-cut, page of type, etc. A mould or impression of the type or coin is first made in wax, or other suitable material. These moulds are, of course, the reverse of the original, and as they do not conduct electricity, have to be coated with graphite. This thin coating lines the mould with conducting material so that the current can get to every part of the mould. These are then hung upon the cathode in a bath of copper sulphate as described in § 82. The electric current which passes through the vat deposits a thin layer of metallic copper next to the graphite. When this copper gets thick enough, the wax is melted away from it, leaving a thin shell of copper, the side next to the graphite being exactly alike in shape to the type, but made of copper. These thin copper sheets are too thin to stand the pressure necessary on printing presses, so they are strengthened by backing them with soft metal which fills every crevice, making solid plates about ¼ in. thick. These plates orelectrotypesare used to print from, the original type being used to set up another page.

84. Polarization.It has been stated that a simple cell polarizes rapidly on account of hydrogen bubbles that form upon the copper plate. They tend to send a current in the opposite direction to that of the main current, which is thereby weakened.

Fig. 81.

85. Electromotive Force of Polarization.It has been shown, Fig. 71, that water can be decomposed by the electric current. Hydrogen and oxygen have a strong attraction or chemical affinity for each other, or they would not unite to form water. This attraction has to be overcome before the water can be decomposed. As soon as the decomposing current ceases to flow, the gases formed try to rush together again; in fact, if the water voltameter be disconnected from the cells and connected with a galvanoscope, the presence of a current will be shown. This voltameter will give a current with an E. M. F. of nearly 1.5 volts; so it is evident that we must have a current with a higher voltage than this to decompose water. ThisE. M. F., due to polarization, is called the E. M. F. of polarization.

86. Secondary or Storage Batteries, also calledaccumulators, do not really store electricity. They must be charged by a current before they can give out any electricity. Chemical changes are produced in the storage cells by the charging current just as they are in voltameters, electroplating solutions, etc.; so it is potential chemical energy that is really stored. When the new products are allowed to go back to their original state, by joining the electrodes of the charged cell, a current is produced.

Fig. 81 shows two lead plates, A and B, immersed in dilute sulphuric acid, and connected with two ordinary cells. A strong current will pass through the liquid between A and B at first, but it will quickly become weaker, as chemical changes take place in the liquid. This may be shown by a galvanometer put in the circuit before beginning the experiment. By disconnecting the wires from the cells and joining them to the galvanometer, it will be shown that a current comes from the lead plates. This arrangement may be called a simple storage cell. Regular storage cells are charged with the current from a dynamo. (See "Study," Exp. 151.)

Fig. 82.

The first storage cells were made of plain lead plates, rolled up in such a way that they were close to each other, but did not touch. These were placed in dilute sulphuric acid. They were charged in alternate directions several times, until the lead became properly acted upon, at which time the cell would furnish a current.

A great improvement was made in 1881, by Faure, who coated the plates with red lead.

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