=275. Storage Batteries.= Storage batteries differ from primary batteries in that current must be supplied to them from some outside source, such as a dynamo. They can be recharged again after the current in them has been used (Fig. 154). [Illustration: FIG. 153. Primary battery.] [Illustration: FIG. 154. Storage battery.] The engines for private homes where a light plant is used are adjusted to charge batteries at the proper rate--but the owner should charge these batteries at regular intervals. They can be charged only by direct current. Never allow the storage battery to run down to a voltage lower than 1.15 per cell. This reading is taken from the voltmeter supplied with the plant. Storage batteries should be tested by a hydrometer for the specific gravity of the electrolyte or liquids in them. Instructions for this and for correcting the specific gravity accompany the plant. Take care to preserve them. Dynamos for home use are almost automatic. Run the dynamo to renew the batteries when using electric irons or other devices calling for more current than the lighting fixtures. Each plant is designed to carry a certain load of equipment. Exceeding this, damages the plant. Place electric motors and dynamos in a dry, cool, clean place. =276. Some Uses for Electric Motors.= Motors are now used on sewing machines, washing machines, dish washers, vacuum cleaners, wringers, fans, refrigerating systems, pumps, grinders, freezers, churns and separators. They are made either for direct or alternating current. When purchasing a motor, be sure to designate the type of current with which it is to be used. Select motors of the right size to operate the machine. It costs more to operate a large motor on a small device than a small motor. =277. Definition Tables.= A British thermal unit is the amount of heat required to warm one pound of water one degree Fahrenheit. The flash point of an oil is that temperature at which it will form an inflammable vapor. The accompanying table shows amount of heat generated from a number of sources. The total heat in a gallon of kerosene is greater than that in a gallon of gasoline because the kerosene is heavier than the gasoline. A gallon of gasoline will give on an average but about five-sixths as much total heat as a gallon of kerosene. This is approximately true, whether the heaviest grades of kerosene are compared with the heaviest grades of gasoline, or the lightest grade of kerosene is compared with the lightest grade of gasoline. Distillate is the refuse left from the distillation of petroleum. The flash point of kerosene may be between 70 and 150 degrees Fahrenheit, depending upon the grade. For illuminating purposes, do not use kerosene with the flash point lower than 120 degrees Fahrenheit. The flash point of gasoline is 10 to 20 degrees Fahrenheit; that is, gasoline will form an imflammable vapor at temperatures as low as this.