each side of every positive. The negative plates are all connected together, and so also are the positive, the object of having a large number of plates being, of course, to increase the capacity of the cell, and to reduce its internal resistance, without resorting to the use of inconveniently large plates. Each grid measures about 8×9 inches, and is inch thick, the weight being about 5 lbs.

The lead grids are provided with massive lugs for the purpose of connection, the lugs of the positive plates being all melted or cast on to one leaden strip or band, the lugs of the negative plates being similarly secured to another strip. The distance between two adjacent plates is about a quarter of an inch, which is usually sufficient to allow pieces of the plates or paste that may become detached to fall to the bottom of the cell. Bent strips or forks of ebonite, celluloid, or other suitable material, are placed between the plates to keep them apart and prevent internal contact. The negative plates in each cell are also held rigidly together by means of two stout strips of lead melted on to solid extensions from the lower edges, two others being also secured to the sides of the plates about half-way up. These connecting strips, one of which is shown at the left-hand side of the figure, serve as a further means of keeping the negative plates in position. The bottom strips rest on slabs or strips of paraffined or varnished wood, so as to support them at a height of about 1 inch above the bottom of the containing cell, affording thereby plenty of room for any scales or pellets that may fall to the bottom to lie clear of the plates. Lugs cast on to the edges of the positive plates rest in small ebonite shoes, which are supported by the side-strips of lead attached to the negative plates. The positives are also connected together across the top by the substantial lead strip shown a little to the right of the middle of the upper edges of the plates. The connecting strip to be seen on the left is melted on to projections from the corners of the plates, consequently they can be readily lifted out of the cell, without necessitating the removal of the negative plates. The containing vessels are best made of stout glass, an opportunity being thus afforded for the proper inspection of the cell, without taking it to pieces or removing the plates. The upper portion of the outer surface of the glass vessel should be coated with wax, vaseline, or some such material, to prevent

N N

'creeping' and the partial short-circuiting of the cells by means of the continuous film of moisture that would otherwise condense over the whole of the external surface. To further ensure good insulation, the cell should be placed on a varnished wooden tray (containing a quantity of sawdust) and supported by insulators of

the so-called mushroom pattern illustrated in fig. 258. The channel in the lower cup contains, as shown in the sectional view of the insulator, a quantity of resin oil or of some other non-evaporating oil, in which the upper cup, coated with shellac varnish, rests. The cells should not be quite in contact, but

fairly close together, and the connections made either by 'burning' or melting the lead strips of adjacent cells together, or by clamping them firmly in a special form of terminal. When the latter method is resorted to, the connection should be painted over with shellac varnish, paraffin wax, or some such impervious

material, to protect the brass against the corrosive action of the acid, which, when the cell is well charged, sprays up considerably. The positive pole of one cell is, of course, connected to the negative of the next, and so on. The positive connecting strips are usually painted red for the purpose of distinction. All leading wires should be as short and of as low resistance as possible, so as

to avoid unnecessary waste of energy in overcoming the resistance of the connections. The resistance of secondary cells is very low, the current obtained from them is usually heavy, and it often happens that the connections offer more resistance than do the cells themselves.

Fig. 259 illustrates a very useful form of cell specially con

FIG. 261.

structed for train lighting. It is enclosed, like other classes of cells intended to be carried in a vehicle, in a teak box lined with lead, and contains, to suit different requirements, either nine or fifteen plates separated by celluloid forks. Sometimes, in order to further reduce the risk of contact and short-circuiting between adjacent plates, thin perforated celluloid sheets are interposed between them. Connection between adjacent cells is of course made externally, rods attached to the connecting strips passing through the covers for the purpose. Connecting rings fit on to these rods, through slots in which wedges are driven to secure good electrical contact, the rings of adjacent cells being joined together by stout wires or rods. Cells are also made specially for tramcar driving, these being, for very obvious reasons, constructed as light as they practically can be, and enclosed in teak or ebonite boxes. They are made in four different sizes conFig. 260 illustrates plates in lead-lined

taining as many different numbers of plates. the method of enclosing the large L type

teak boxes, for purposes where a heavy current is required. The peculiar shape of the forks in this case and in that illustrated in fig. 259 should be noticed.

Another and very important type of the Electrical Power Storage Co.'s cell is that illustrated in fig. 261, and known as the K or high discharge type. The positive plates are considerably thicker than the negative, and are connected together and supported by two lead bars of large dimensions which rest on insulating

saddle-pieces placed on the upper edges of the negative plates. These two bars are joined by an angular piece of lead, so that the connection from cell to cell is made at the middle of each end of the containing vessel instead of at the sides, as in fig. 257.

The negative plates stand on prolongations or feet at each of the bottom corners of each plate, the plates being joined together at the foot by lead bars of similar dimensions or area to those joining the positive plates, while at the top corners the negative plates have a projection at the side, which is also utilised for joining the plates together by bars of lead. At one end of the bars joining the feet of the negative plates a cross-bar and lug, in the form of an inverted tee-piece, forms the connecting lug, which leaves the containing vessel at the middle of one side and in such position as will enable it to meet the angular connecting lug from the positive plates of the next cell.

The separation between the respective plates is obtained in the usual manner by ebonite forks. There are small niches in the top edge of the positive plates in which. the forks are intended to rest.

In fig. 262 is shown, full size, a portion of one of the positive plates of these cells. It consists of a mas

FIG. 262.

sive lead casting, with a number of thin horizontal ridges on both faces. These ridges are inclined upwards and are packed with the usual minium paste. A portion of one of the negative grids is illustrated in fig. 263. This grid consists of a comparatively light lead casting with a number of small projecting prongs, the function of which is to keep the small litharge pellets in position. When the discharge rate from a secondary cell is excessive, the active material expands, and there is therefore with cells of the L type a tendency to dislodge it. This, as will be seen presently, results in the distortion of the plates and the disintegration or splitting of the pellets, which consequently fall out of the grid. As the risk of this action taking place with the K plate is obviously