to form an element or plate group, are lead-burned to soft-lead busbars.

The total number of cells required is determined by the maximum busbar voltage permissible at the end of a high-discharge rate. For example, if a battery is installed of capacity enough to carry a maximum peak load for ten minutes a voltage of 100 each side of a three-wire system might be considered satisfactory. This would call for 75 cells each side. If at any time during the day it is necessary to float the battery at a voltage as low as 115 volts, the main battery would consist of 55 cells each side, leaving an auxiliary battery of 20 end-cells on each side. These end-cells are brought into action as needed by special switches.

In arranging a battery-room for such a large battery, special attention is given the floor construction, because drainage is important. Ventilation must be exceptionally good, and all exposed metal work should be protected by acid-proof paint. Concrete has been used for battery-room floors, and is satisfactory if care is taken to flush it with water frequently to wash away any electrolyte that may have accumulated. Hard-burned tile or vitrified brick is much more suitable. The floor should be laid on a concrete foundation, of sufficient strength to carry the weight. The slope should be such as to allow for positive drainage, and the floor covered with asphaltum felt. Spaces of 1/4 inch are left between the bricks or tiles, these spaces being grouted with asphalt compound. An exhaust ventilating system is almost an essential if the battery is used much. The fan parts should be of bronze; and air from the room should be filtered through an air-box having perforated lead screen to eliminate acid spread. It is said that air in a battery-room should be changed completely four times an hour during the gassing period of the charge.

Storage Batteries for Draw-Bridge Operation.-An unfailing source of power is an absolute necessity where draw-bridges are operated by electric motors, as most of them are. This applies especially to railway bridges, where any failure of the power supply would seriously interrupt travel on either the waterway or railway. Vessels have been badly injured due to failure to open a draw. Bridges may be operated by separate power plants, consist

Fig. 88.-Calumet & Western Indiana Railroad Bridge in Open and Closed Positions.

ing of dynamos driven by internal-combustion engines, or they may take their power from a trolley or lighting system. In either case, an auxiliary storage-battery installation insures absolutely reliable service, even in event of failure of the main source of current. This is of special value where a bridge is over a widely used stream. The main principles of operation and the requirements are practically the same as obtain when storage batteries are used for stand-by service. A typical installation, where the movable span of the bascule pattern is 186 feet long and weighs 1,100,100 pounds itself, in addition to a 3,000,000-pound concrete counterbalance, consists of two batteries, one to furnish power for the bridge motors, the other for signals and lighting. The larger battery consists of 120 type F11 "Chloride Accumulator" elements in lead-lined wooden tanks. Each element has a normal capacity. of 400 ampere-hours at 240 volts. The general arrangement of the tanks in the battery-room is shown at Fig. 87. The views at Fig. 88 show the bridge in open and closed positions.

Edison Storage-Battery Mine Lamp. The greatest danger in mines to-day is from the use of unprotected flames wherever the deadly fire damp is likely to be encountered. Artificial light is an essential which formerly could not be obtained with safety unless provided in such minute quantity as to seriously curtail production. The Davy safety oil lamp is well known and has been widely used, but electric lighting gives much superior results. Much ingenuity has been shown in trying to adapt portable electric lamps to this work, but the stumbling-block always has been the production of current for their operation. The only practical source of energy is naturally some sort of battery, and many attempts have been made to modify the old types of cells so that they would serve the purpose satisfactorily. The advocates of the primary battery soon found that the inherent defects of this type were greatly magnified when an endeavor was made to produce a portable form of small size and weight with sufficient capacity to keep an electric lamp burning any considerable time. Aside from this, the electrical energy is produced in a primary battery by the consumption of the zinc plates, so that there was constant expense and trouble for their renewal. The primary cell was early elimi

nated from serious consideration, and experiments made with secondary or storage batteries have until lately been far from successful. The Edison mine lamp, shown at Fig. 89, is considered a practical solution of this problem, because of the "meddle-proof" qualities of the Edison alkaline battery. The cells used are the same in principle as the nickel-iron alkaline batteries developed for other purposes, but are small and very light.

The cells fit snugly into a light case of rust-proof steel, which is primarily a box in which to carry the battery. There is no insulation between the cells, and the case and the contact springs on the cell poles hold the battery securely when the cover is in place.

Fig. 89.-The Edison Storage Battery Mine Lamp Outfit and

How It is Used.

The cover has a separable hinge, which permits its entire removal when open and facilitates charging the batteries in "banks." Covers and cases, equipped with self-contained locks, are interchangeable. The two cells are connected in series, the positive pole of one and the negative of the other being grounded to their containers, and the containers connected together. The free terminals carry the spiral contact springs, which press against nickeled-steel contact plates in the cover. The contact plates are insulated from the cover and receive the cable terminals. A twin-conductor, rubber-covered cable connects the battery to the cap lamp. At each end the cable is thoroughly armored, preventing injury from sharp bending. While lamp and reflector are being carried in the hand or at other times the armor takes up all the weight, so there is no possibility of strain coming upon the wires at the terminals. An ingenious arrangement permits the easy replacement of the cable should it be cut or otherwise injured in service. The cap lamp consists of a nickel-plated brass reflector provided with a hook to fit into the regulation miner's cap. A tungsten lamp is forced into a spring socket by means of a clip at its tip in such a way that if the lamp be broken the base is immediately disconnected and the lamp extinguished. This safety feature has been thoroughly tested by the Bureau of Mines and unqualifiedly approved under Schedule 6A.