To determine the ampere hours from this table, multiply the total number of lamp hours by the current in amperes required for each lamp. As 10, 15, and 20-watt tungsten lamps require .33, .50 and .67 amperes, respectively at 30 volts pressure, the above requirements in ampere hours would be 12, 17-1/2, or 24 ampere hours, according to the size of lamp chosen. This gives the average current consumption for one night. If it is desired to charge the battery twice a week on the average, multiply the number of lamp hours by 4, to get the size of battery required. The foregoing illustration is not intended to indicate average light requirements for farms, but is given merely to show how a farmer may figure his own requirements. In some instances, it will be necessary to install a battery of 120 or more ampere hours, whereas a battery of 40 or 60 ampere hours would be quite serviceable in other instances. It all depends on how much light you wish to use and are willing to pay for, because with a storage battery the cost of electric light is directly in proportion to the number of lights used. As a general rule, a larger generator and engine are required for a larger battery--although it is possible to charge a large battery with a small generator and engine by taking more time for the operation. _How to Charge a Storage Battery_ Direct current only can be used for charging storage batteries. In the rare instance of alternating current only being available, it must be converted into direct current by any one of the many mechanical, chemical, or electrical devices on the market--that is, the alternating current must be straightened out, to flow always in one direction. A shunt-wound dynamo must be used; else, when the voltage of the battery rises too high, it may "back up" and turn the dynamo as a motor, causing considerable damage. If a compound dynamo is already installed, or if it is desired to use such a machine for charging storage batteries, it can be done simply by disconnecting the series windings on the field coils, thus turning the machine into a shunt dynamo. The voltage of the dynamo should be approximately 50 per cent above the working pressure of the battery. For this reason 45-volt machines are usually used for 30 or 32-volt batteries. Higher voltages may be used, if convenient. Thus a 110-volt dynamo may be used to charge a single 2-volt cell if necessary, although it is not advisable. _Direction of Current_ Electricity flows from the positive to the negative terminal. A charging current must be so connected that the negative wire of the dynamo is always connected to the negative terminal of the battery, and the positive wire to the positive terminal. As the polarity is always marked on the battery, there is little danger of making a mistake in this particular.