In order to make apparent this density of the electrolyte without the necessity of its measurement with a hydrometer, one cell of the battery is provided with a gage as that of Fig. 262. This is an enlargement of the end of the jar in which floats a hollow glass ball of such weight that it will at any time indicate by its position the relative density of the solution. When the cell is charged the ball stands at the top of the gage and indicates a density 1220; when discharged it is at the bottom and expressed by its position a density of 1185. The electrolyte densities are the indicators of the conditions of charge. The ball by its position shows at a glance the quantity of electricity in the battery.

The voltage usually employed in household electric plants is that of a battery composed of 16 cells. Since the normal voltage of a storage cell is 2 volts such a battery joined in series is 32 volts. This voltage for the purpose fulfills all ordinary conditions and is generally employed. A battery of 16 cells, of 80-ampere-hour capacity, will deliver current of 1 ampere for 80 hours at 32 volts intensity. A 20-watt lamp on a 32-volt circuit requires 2/3 ampere for its operation. The battery will, therefore, keep lighted one such lamp for 96 hours, or four 20-watt lamps may be lighted continuously for 24 hours, or eight lamps for 12 hours, before recharging.

Aside from its ability to supply the required light for the average home, it furnishes energy sufficient for heating a flat-iron or other heating apparatus, to operate motors for pumping water, driving a washing machine or any other of the domestic requirements.

Such plants are made in sizes to suit any condition of requirement. In large establishments a larger motor generator and battery will be necessary with which to generate and store the required electricity but in any case suitable apparatus is to be obtained to meet any requirement of light, heat or power developed.

Fig. 262. - Electric storage cell.

National Electrical Code

The details governing the size, the manner of placing and securing wires in buildings is included in the regulations published by the National Board of Fire Underwriters as the National Electric Code. Likewise the mechanical construction of all apparatus dealing with electric distribution is definitely specified so that manufacturers furnish reliable materials for all requirements. In the specifications for furnishing buildings with the use of electricity, descriptions are made of the desired types and styles of the switches and various other fixtures to suit the requirements.