Читать книгу Harper's Electricity Book for Boys - Joseph H. Adams - Страница 7

In order to generate electricity it is necessary to employ cells, batteries, or dynamos. Since the construction and operation of a dynamo is somewhat intricate, it will be better to start with the simpler methods of electric generation, and so work up to the more complicated forms. For small apparatus, such as electric bells and light magnets and motors, the zinc-carbon-sal-ammoniac cell will answer very well; but for larger machinery, where more current is required, the bluestone and the bi-chromate batteries will be found necessary.

SIMPLE BATTERY ELEMENTS

A simple and inexpensive cell may be made from electric-light carbons, with the copper coating removed, and pencils of zinc, such as are used for electric-bell batteries and which can be purchased for five cents each. Copper wire is to be bound around the top of each pencil of carbon and zinc, and firmly fastened with the pliers, so that it will not pull off or become detached. It will be well to cut a groove with a file around the top of both the carbon and zinc, into which the wire will fit. The elements should then be clamped between two pieces of wood and held with screws, as shown in Fig. 1. A more efficient carbon pole is made by strapping six or more short carbon pencils around one long one, as shown in Fig. 3. The short pieces of electric-light carbons are bound to the longest carbon with heavy elastic bands, or cotton string dipped in paraffine or wax, to make the cotton impervious to water and the sal-ammoniac solution.

Another arrangement of elements is shown in Fig. 2, where a zinc rod is suspended between two carbons, the carbons being connected by a wire that must not touch the zinc.

A fruit-jar, or a wide-necked pickle-bottle, may be employed for a cell, but before the solution is poured in, the upper edge of the glass should be coated with paraffine. This should be melted and applied with a brush, or the edge of the glass dipped in the paraffine.

The solution is made by dissolving four ounces of sal-ammoniac in a pint of water, and the jar should be filled three-fourths full. In this solution the carbons and zinc may be suspended, as shown in the illustration (Fig. 4) of the sal-ammoniac cell. The wood clamps keep the carbon and zinc together, and the extending ends rest on the top of the jar and hold the poles in suspension. Plates of zinc and carbon may be clamped on either side of a square stick and suspended in the sal-ammoniac solution, as shown in Fig. 5, taking care, however, that the screws used for clamping do not touch each other.

If one cell is not sufficiently powerful, several of them may be made and coupled up in series—that is, by carrying the wire from the zinc of one to the carbon of the next cell, and so on to the end, taking care that the wire from the carbon in the first cell and that from the zinc of the last cell will be the ones in hand, as shown in Fig. 6. This constitutes a battery. Be sure and keep the ends of the wire apart, to prevent galvanic action and to save the power of the batteries.

This battery is an excellent one for bells and small experimental work, and when inactive the zincs are not eaten away (as they would be if suspended in a bi-chromate solution), for corrosion takes place only as the electricity is required, or when the circuit is closed. A series of batteries of this description will last about twelve months, if used for a bell, and at the end of that time will only require a new zinc and fresh solution.

The cell in which the plates shown in Fig. 5 are used may contain a bi-chromate solution; and for experimental work, where electricity is required for a short time only, this will produce a stronger current. But remember that the solution eats the zinc rapidly, and the plates must be removed as soon as you have finished using them.

The bi-chromate solution is made by slowly pouring four ounces of commercial sulphuric acid into a quart of cold water. This should be done in an earthen jar, since the heat generated by adding acid to water is enough to crack a glass bottle. Never pour the water into the acid. When the solution is about cold, add four ounces of bi-chromate of potash, and shake or mix it occasionally until dissolved; then place it in a bottle and label it:

BI-CHROMATE BATTERY FLUID

POISON

Before the zincs are immersed in the bi-chromate solution they should be well amalgamated to prevent the acid from eating them too rapidly.

The amalgamating is done by immersing the zincs in a diluted solution of sulphuric acid for a few seconds, and then rubbing mercury (quicksilver) on the surfaces. The mercury will adhere to the chemically cleaned surfaces of any metal except iron and steel, and so prevent the corroding action of the acid. Do not get on too much mercury, but only enough to give the zinc a thin coat, so that it will present a silvery or shiny surface.

A two-fluid cell is made with an outer glass or porcelain jar and an inner porous cup through which the current can pass when the cup is wet. Fig. 7.

A porous cup is an unglazed earthen receptacle, similar to a flower-pot, through which moisture will pass slowly. The porous cup contains an amalgamated plate of zinc immersed in a solution of diluted sulphuric acid—one ounce to one pint of water. The outer cell contains a saturated solution of sulphate of copper in which a cylindrical piece of thin sheet-copper is held by a thin copper strap, bent over the edge of the outer cell. A few lumps or crystals of the copper sulphate, or bluestone, should be dropped to the bottom of the jar to keep the copper solution saturated at all times. When not in use, the zinc should be removed from the inner cell and washed off; and if the battery is not to be employed for several days, it would be well to pour the solutions back into bottles and wash the several parts of the battery, so that it may be fresh and strong when next required. When in action, the solutions in both cups should be at the same level, and be careful never to allow the solutions to get mixed or the copper solution to touch the zinc. Coat the top of the porous cell with paraffine to prevent crystallization, and also to keep it clean. Take great care, in handling the acid solutions, to wear old clothes, and do not let the liquids spatter, for they are strong enough to eat holes in almost anything, and even to char wood. The two-fluid cells are much stronger than the one-solution cells, and connected up in series they will develop considerable power.

For telegraph-sounders, large electric bells, and as accumulators for charging storage-batteries, the gravity-cell will give the most satisfactory results. The one shown in Fig. 8 consists of a deep glass jar, three strips of thin copper riveted together, and a zinc crow-foot that is caught on the upper edge of the glass jar. These parts will have to be purchased at a supply-house, together with a pound or two of sulphate of copper (bluestone).

To set up the cell, place the copper at the bottom and drop in enough of the crystals to generously cover the bottom, but do not try to imbed the metallic copper in the crystals; then fill the jar half full of clear water. In another jar dissolve two ounces of sulphate of zinc in enough water to complete the filling of the jar to within two inches of the top; then hang the zinc crow-foot on the edge of the jar so that it is immersed in the liquid and is suspended about three inches above the top of the copper strip. The wire that leads up from the copper should be insulated with a water-proof coating and well covered with paraffine. A number of these cells may be connected in series to increase the power of the current, and for a working-battery this will show a high efficiency. Note that at first the solutions will mingle. To separate them, join the two wires and start the action; then, in a few hours, a dividing line will be seen between the white, or clear, and the blue solutions, and the action of the cell will be stronger. After long-continued use it may be necessary to draw off some of the clear zinc sulphate, or top solution, and replace it with pure water. The action of the acids reduces the metallic zinc to zinc sulphate and deposits metallic copper on the thin copper strips, and in this process an electrical current is generated.