This is simply a chloride of silver cell, with caustic potash instead of chloride of zinc. Two of the cells, contained in ebonite cases buckled to the belt of the performer, keep a star light going, and the light is readily controlled by the wearer. Each cell consists of a zinc plate bent into the form of a U, and holding in its inside a plate of silver coated with chloride of silver. The zinc plate forms one pole of the cell and the silver the other. A solution of caustic potash - 75 parts potash to 100 water - is poured in, and as a porous diaphragm, the chloride of silver is covered with parchment paper. The vessel is of ebonite, with closed mouth, which, however, is opened when fresh liquid has to be put into the cell. This is necessary after every hour's run, and the chloride of silver has to be replaced after 3-4 hours' use. The cell is thus expensive; but this is of minor importance in theatrical work as compared with its small size and weight - 3 1/2 oz.

Sloane's

This battery, Fig. 103, is a very efficient and simple form for open or closed circuit work. It represents a favourite and recent type for such cells, and can be put together with the minimum number of tools and appliances. The cover is made of wood. If a circular vessel is used, the cover should be cut in a circle equal in diameter to the outside of the jar, and a shoulder'should be formed to hold it in place and prevent lateral motion. Any number of holes, according to the size, are bored through it, one set for the reception of the carbons and the others for the zincs. Care should be taken to bore these holes truly vertical to the plane of the cover, and the bit used should make a hole of exactly the right size to fit the carbon and zincs respectively. The fit must be a very tight one, so that the rods have to be driven into their places with a mallet or hammer.

For the positive elements, zinc rods, such as sold for the Leclanche* battery, are used. Such rods can be bought 6-8 ft. long and of uniform diameter. Pieces are cut off of the proper length, a cold chisel, hack saw, or file being used. A very easy way of dividing the rod is with mercury. A fine groove is filed around it. A globule of mercury is placed in a saucer with a little dilute sulphuric acid. A thin slip of zinc, or a strip of galvanised iron, is dipped in the mercury. Some adheres to it. This is then drawn around the cut, so as to fill it with mercury and amalgam. Then the rod is broken off, either in the hand or in a vice. It becomes almost as brittle as a pipe stem. This process is hardly to be recommended for the upper ends of the zincs. These have to be soldered, and the mercury interferes with the operation to some extent.

For negative elements, electric light carbons are used. The copper is dissolved off by nitric acid, they are washed, dried, cut to the proper length by a saw or cold chisel, and their upper ends are soaked in hot paraffin.

Both elements are now driven into their respective places. With each carbon a slip of copper Jin. wide is also introduced, and lies alongside, pressed hard against it and projecting about as much below the cover. As shown, a wire is carried around the outer circle of the carbons, and is soldered to the copper strips. If a central carbon has been used, a special connection is soldered to it and to the main wire.

Selenium element

Selenium element.

The end of the wire it carried of through a hole in the cover. A second wire is soldered to the zincs, this piece lying on the upper surface of the cover. Concentrated hydrochloric acid it the but flux for the zincs. If desired, the projecting end of the zinc connection may be secured to the wood by a staple. This is not necessary if the soldering is

To amalgamate the zincs, a strip of galvanised iron is far the best instrument. The end of such a piece, which may be 2 in. by J in., is bent into a hook, so as to fit the zinc rods. This is dipped into the globule of mercury as it lies under a little dilute acid, and is rubbed up and down the reds. If the mercury does not take hold at once, the lines and carbons may be dipped nearly to the level of the cover in dilote sulphuric acid. After a few minutes' immersion, the zinc will be ready to amalgamate, and the rods will shine like silver after a few minutes' rubbing with the galvanised iron and mercury. The soldering may of course be dispensed with. Instead of strips of copper, the ends of some pieces of wire may be flattened and driven into the holes alonp with the carbons and zincs. By twisting together the ends of these, zinc connections and copper connections separately, the battery will work per-fectlyifcare is taken to avoid abort-circuiting. When it is mode in a harry, for temporary use only, tha paraffining of the carbons may be dispensed with, and the copper may be left upon their upper ends.

The wires may be soldered directly to this, although such connection is rather weak.

Single fluid battery.

Single fluid battery.

For bichromate solution, 2 1/2 oz. bichromate of potash in fine powder are shaken up in 10 fl. oz. water. To this 2 1/4 fl. oz. sulphuric acid are added slowly with constant stirring. Great care should be taken in pulverising the bichromate of potash, as it causes ulcers if inhaled. For open circuit work, a solution of sal ammoniac may be used. The ends of burned-out carbons, such as are thrown away by the lamp attendants, answer perfectly for the smaller sizes of this battery.

Water

Strips of zinc and copper, each 2 in. wide, are soldered together along their edges so as to make a combined strip of a little less than 4 in. wide, allowing for the overlapping. It is then cut by shears into pieces about 1/4 in. wide, each composed of half zinc and half copper. A plate of glass, very thick and 1 ft. or less square, is heated and coated with shellac about 1/8 in. thick. The strips of copper and zinc are bent into the shape of the letter U, with the branches about 1/4 in. apart, and are heated and stuck to the shellac in rows, the soldered portion being fixed in the shellac, and the two branches standing up in the air, so that the zinc of one piece comes within 1/16 in. of the copper of the next one. A row of 10 in. long will thus contain about 30 elements. The rows can be about 1/10 in. apart, and therefore in a space 10 in. square nearly 800 elements can be placed. The plate is then warmed carefully so as not to crack, and a mixture of beeswax and rosin, which melts more easily than shellac, is then poured on the plate to a depth of 4 in., to hold the elements in place. A frame of wood is made around the back of the plate with a ring screwed to the centre, so that the whole can be hung up with the zinc and copper elements below.

When required for user lower so as to dip the tips of the elements into a pan of water, and hang up again. The space between the elements being 1/16 in. will hold a drop of water which will not evaporate for possiibly an hour. Thus the battery is in operation in a minute, and is perfectly insulated by the glass and 5 cement. This is the form I have used; but the strips might better be soldered face to face along one edge, cut up, and then opened. (Prof. Rowland.)