The tungsten lamp, requiring only one-third as much electric current as the carbon lamp, for the same amount of light, reduces the size (and the cost) of the storage battery in the same degree, thus bringing the storage battery within the means of the farmer. Some idea of the power that may be put into a small storage battery is to be had from the fact that a storage battery of only 6 volts pressure, such as is used in self-starters on automobiles, will turn a motor and crank a heavy six-cylinder engine; or it will run the automobile, without gasoline, for a mile or more with its own accumulated store of electric current. _The Low Voltage Battery_ The 30-volt storage battery has become standard for small lighting plants, since the introduction of the tungsten lamp. Although the voltage of each separate cell of this battery registers 2.5 volts when fully charged, it falls to approximately 2 volts per cell immediately discharging begins. For this reason, it is customary to figure the working pressure of each cell at 2 volts. This means that a 30-volt battery should consist of at least 15 cells. Since, however, the voltage falls below 2 for each cell, as discharging proceeds, it is usual to include one additional cell for regulating purposes. Thus, the ordinary 30-volt storage battery consists of 16 cells, the last cell in the line remaining idle until the lamps begin to dim, when it is switched in by means of a simple arrangement of connections. This maintains a uniform pressure of 30 volts from the beginning to the end of the charge, at the lamp socket. We saw in earlier chapters that the 110-volt current is the most satisfactory, under all conditions, where the current is to be used for heating and small power, as well as light. But a storage battery of 110 volts would require at least 55 cells, which would make it too expensive for ordinary farm use. As a 30-volt current is just as satisfactory for electric light, this type has become established, in connection with the battery, and it is used for electric lighting only, as a general rule. Batteries are rated first, as to voltage; second, as to their capacity in ampere hours--that is, the number of amperes that may be drawn from them in a given number of hours. Thus, a battery rated at 60 ampere hours would give 60 amperes, at 30 volts pressure, for one hour; 30 amperes for 2 hours; 15 amperes for 4 hours; 7-1/2 amperes for 8 hours; 3-3/4 amperes for 16 hours; etc., etc. In practice, a battery should not be discharged faster than its 8-hour rate. Thus, a 60-ampere hour battery should not be drawn on at a greater rate than 7-1/2 amperes per hour.