when batteries of the alkaline type are used. Lead batteries may be placed under a hood and the gases allowed to escape to, the outside through a suitable vent if they are installed in the same room as apparatus that may be affected by fumes evolved during the charging process.

For railway signaling the alkaline battery has great advantages by reason of its electrolyte. There being no corrosive fumes or vapors produced, the batteries may be placed anywhere in the signal tower. There is no danger of injuring the most delicate apparatus, and many installations of such batteries are in the same room as and in close proximity to relays, generators, air-compressing machinery, etc.

The Stand-by Battery.-The storage battery is used in many central stations as an emergency source of current to cope with unusual current requirements or to supply electricity for a brief time in event of damage to the dynamos or their prime mover. Among some of the conditions that would call for discharge of a stand-by battery would be interruption of current supply, due, for instance, to low steam or belt breakage in a steam plant or collection of ice in the forebay of a hydraulic generating plant. A breakdown of the generating machinery, putting some of the dynamos out of commission, thus reducing the plant capacity, would necessitate the use of the emergency current. An accidental opening of a transmission line or unexpected increase of load, such as is caused by everyone turning on the electric lights because of a sudden darkening in the daytime prior to a rainstorm, will also draw on the reserve source.

In the early applications of storage batteries to central station. service one of the main objects sought was to improve the daily load factor in a steam plant by discharging the battery during the evening peak or period of heavy demand and recharging it during the hours of lighter load. This will result in a marked improvement in plant efficiency because power costs less if produced at a uniform rate. Where the peak is of short duration, the cost of a battery will be less than the added steam equipment it displaces, and as it conduces to greater economy of current production, this is a clear gain. When a charged storage battery is

Fig. 85.-How Stand-by Battery Helps on Peak Load Work.

available as an auxiliary source of energy, it is always ready for emergency use, and this advantage is one of great value and can hardly be considered fairly on a purely pecuniary basis. The chart at Fig. 85 shows the operation of a battery in daily peak-load work. That at Fig. 86 shows how the output of a stand-by battery helped to handle an unexpected lighting load on a central station due to a particularly heavy thunderstorm. These curves are merely graphic records of the amount of current used under certain conditions and in a given time, and are easily understood.

A stand-by battery uses rugged plate construction and is always 'composed of very large-size cells. Type H Exide plates, which are widely used in the larger installations, are 31 inches high by 155/16 inches wide. The grids are castings of leadantimony alloy and are provided with very heavy connecting lugs. When it is considered that a plate of this size may be called upon to discharge 600 amperes or more for several minutes, it will be realized that great care must be taken in proportioning the plates. The plates are hung from the cell tops by the plate lugs, which rest on vertical pieces of heavy glass arranged on either side of the tank, suitably notched to receive the plate lugs. The glass plates rest upon the reinforced lead lining at the bottom of the cell. A space of 3/4 inch is left between the outside negative plate and side of the tank at one end to permit the taking of hydrometer readings. An instance of the large size of the cells is the sediment space allowed, which is 12 inches in a type H cell.

A tank suitable for an Exide element having a capacity of 3,000 amperes for one hour measures about 225% inches long by 2111⁄2 inches wide. For a capacity of 6,000 amperes at the hour rate the length is increased to three feet, and for 9,000 amperes the length is nearly five feet. The height of such a cell from the floor to the busbar is about 512 feet, or high enough so the average man can barely look into the cell. The tanks are built of specially selected yellow pine, put together with glued, dovetailed and doweled joints. No nails or metallic fastenings of any kind are used. The lumber used is of sufficient strength to be entirely self-supporting. These tanks are treated with two coats of acidresisting paint inside and out.

The tank linings are of pure sheet lead, the lap seams being burned with the hydrogen flame. The upper edges of the lining extend over the edges of the tank and down outside clear of the tank faces. Drip points are provided, so spaced that they come between the tank supports. Good insulation of the cells is an essential. The Exide insulator consists of a glass body surrounded by an inverted petticoat, this forming an annular trough partly

filled with oil, the whole being covered with a lead cap extending down around the sides but out of contact therewith. Each oil insulator rests on a truncated cone or pedestal of earthenware or a composition not affected by acid. Each cell is covered with a plate of heavy glass, which serves to condense the acid spray. All conductors are of specially heavy section lead-coated copper bars, firmly bolted together.. The plates of each cell, joined together