Daniel's Battery (generally called the constant battery, on account of its remaining in action for a great length of time) consists in dividing the ordinary cell into two by inserting a porous pot or other porous diaphragm. Fig. 15 shows a section of an ordinary Daniel cell.

In the glass cell is placed a cylinder of copper in a solution of sulphate of copper (blue vitriol); in the porous cell-in a solution of sulphate of zincis placed a solid rod of zinc; the zinc of one cell is connected to the copper of the next.

The action of this battery, by the use of the porous cell, prevents the injurious effect of the evolution of hydrogen in the single-liquid arrangement, the effect of the hydrogen was to deposit oxide of zinc on the copper plate, neutralizing in a great measure the effectiveness of the battery.

A very convenient arrangement of Daniel's battery is shown in Fig. 15 a, in great use amongst the principal telegraph companies.

A trough of wood, coated internally with some insulating and waterproof substance, is divided into ten or twelve cells; these are further subdivided by inserting between each two divisions a porous plate. A plate of zinc and one of copper are soldered together, and placed one on each side of the wooden division; the separate spaces are filled with solutions of sulphates of copper and zinc. The last plate in each trough is fixed down by a screw, to which a wire may be attached.

In fact, there are many arrangements of Daniel's battery, but the principle remains the same.

Groves's Nitric Acid Battery.-In this arrangement (Fig. 16) a zinc cylinder is placed in the outer cell. Inside the cylinder is a porous pot, with a platinum plate for the negative element. The exciting liquids are common nitric acid with the platinum, and a solution of sulphuric acid with the zinc. The action of the battery is very powerful, but it does not remain in order for any length of time. This battery is unpleasant for using in a room, as the fumes from the nitric acid are disagreeably strong.

Smee's Battery is one of those that require only one exciting liquid. On a wooden bar is attached a platinized silver plate, P, with a binding-screw on its upper surface. On each side of the bar are placed two large plates of zinc, kept together by a metal clamp. When required for action, the jar, or guttapercha cell, is charged with common sulphuric acid (1 part to 7 of water); the plates are then dipped in. This arrangement is one very frequently used, and very much so for electrotyping. (Fig. 17.)

P

FIG. 16.

FIG. 17.

Bunsen's Carbon Battery. This is an arrangement very similar to that of Groves, only carbon is used instead of platinum, the exciting liquids being the same. Although much used, it has the same unpleasant disadvantage as the Bunsen; however, the following alteration of exciting liquids does away with that disadvantage, and gives us a battery which lasts for some days in pretty constant action, and has the great recommendation of being cheap.

In the carbon cell, put two parts of a saturated solution of bichromate of potash, and one part of common sulphuric acid: with the zinc, put a saturated solution of common salt. A saturated solution is when the water contains so much in solution that no more crystals can be dissolved.

For the bichromate of potash, dissolve in hot water the crystals (it will be found that about 2 oz. go to a pint), to this add pint of common sulphuric acid. Care must be taken in mixing sulphuric acid. It should be done gradually and quietly, for the great heat evolved by it might break the jug or jar. When cool, charge the cells. (Fig. 18.)

This arrangement is excessively useful for all experiments, especially for

those in connection with induction coils, for which I have used it myself, and find it a great comfort. I certainly recommend it, for various reasons, in

preference to all others.

The small battery I have just used consists of a large jam-pot; a piece of zinc formed into a cylinder, with a wire attached; a porous pot, and a piece of carbon. It is in action at once, and the arrangement simple.

Sulphate of Mercury.-This is again a single-liquid battery, and is very simple. In a glass bottle (Fig. 19) are two elements, consisting of a rod of zinc and a plate of carbon, passing through a porcelain cover, and terminating outside in binding-screws. The bottle is charged with about 3 oz. of bisulphate of mercury; water is then poured in up to nearly the shoulder; the elements are then inserted. This battery is the most useful of the kind for ringing household bells, &c., and for purposes where you only require occasional action, and for no great length of time. Under such circumstances I have known one of these batteries last over twelve months without being touched; it then only required some more bisulphate of mercury.

Bichromate of Potash Battery.-This is a battery that, for experiments, is in great request; it is strong in

action, but does not last so long as could be wished. It consists of carbon for the negative, and zinc for the positive clements, excited by one part of a saturated (see before) solution of bichromate of potash, to about ten parts of sulphuric acid. A convenient form of this battery is shown in Fig. 20, where the centre rod is of zinc, and can be drawn up out of the liquid when the battery is not wanted, in order to prevent

waste.

In the various batteries I have described there are again variations, and further description would become tedious.

In connecting up a few cells into a battery, the arrangement is called one of intensity when the zinc of one cell is connected on to the copper of the next, and so on throughout the series; but when the zincs are all connected together, and the coppers together, the arrangement is called one for quantity.

In the first arrangement, the quantity of electricity evolved in the battery is the same as that evolved

FIG. 2C.

by one pair; but the intensity increases in direct proportion to the number of cells used.

Intensity depends upon the number of cells, and not upon the size of the plates; quantity, on the contrary, depends upon the size of the plates.

If you increase the size of the plates six times, you increase in the same proportion the quantity of electricity evolved.

On joining up the cells (Fig. 21) for quantity, you practically increase the

size of the plates, and consequently obtain proportionately increased quantity; but in this case the intensity is the same as with the one cell.

The accompanying sketch (Fig. 22), showing an elevation of a Bunsen carbon battery, will place the nomenclature of the various parts of a battery

beyond doubt. Make yourself perfectly familiar with these terms, and you will save yourself a great deal of trouble, and be enabled at once to comprehend the manner of carrying out experiments which otherwise you might find a difficulty in grasping.

When you join the two ends of the wires from the battery together, allowing a current of electricity to circulate through, you establish what is termed a "circuit."

One of the great results due from the pile was the discovery of Ersted in 1819. He found that a magnetic needle suspended near a body, placed in the same meridian, through which an electric current was passing, tended to place itself at right angles to that body. He also observed that the deviation of the needle depended upon the quantity of the current, and its direction upon the direction of the

current.

Some few years after this discovery, it was remarked that increased effects were

observed in the needle by increasing the turns the wire took round the needle; for, at first, a wire was passed above or below the needle, and then completely round. Instruments were made with many turns of wire round the needle; this rendered the needle more sensitive to the presence of electricity. These instruments were and are still called Galvanometers: the addition of a scale of degrees to mark the deviation of the needle to the one side or the other was a still further improvement. Instruments so constructed are in daily use; they are now made of various shapes, with vertical or horizontal needles, with various quantities of wire according to the required sensitiveness.

The next discoveries were due to Ampére and Arago, who found that wires when under the influence of electric currents were magnetic, and attracted iron filings, &c. Arago further noticed that iron became temporarily magnetic whilst under the influence of electric currents passed near or around it; steel, under similar circumstances, becoming permanently magnetic.

This was the origin of the grand invention of Electro-magnets, the use of which is now so great, and the principle of which may be found in almost all the important telegraph instruments of the present time. In fact, in telegraphy we depend entirely upon the discoveries of Ersted and Arago for enabling us to produce instruments for detecting electric currents or receiving messages. That a bar of soft iron becomes magnetic may be easily tried by experiment. Round a bar of soft iron wind some wire in the form of a helix (Fig. 24); attach