When the storage battery is charged, and one begins to use its accumulation of energy, the current comes out in the opposite direction from which it entered in charging. In this respect, a storage battery is like a clock spring, which is wound up in one direction, and unwinds itself in the other. With all storage battery outfits, an ammeter (or current measure) is supplied with zero at the center. When the battery is being charged, the indicating needle points in one direction in proportion to the strength of the current flowing in; and when the battery is being discharged, the needle points in the opposite direction, in proportion to the strength of the current flowing out. Sometimes one is at loss, in setting about to connect a battery and generator, to know which is the positive and which the negative wire of the generator. A very simple test is as follows: Start the generator and bring it up to speed. Connect some form of resistance in "series" with the mains. A lamp in an ordinary lamp socket will do very well for this resistance. Dip the two ends of the wire (one coming from the generator, the other through the lamp) into a cup of water, in which a pinch of salt is dissolved. Bring them almost together and hold them there. Almost instantly, one wire will begin to turn bright, and give off bubbles. The wire which turns bright and gives off bubbles is the _negative_ wire. The other is the positive. [Illustration: A rough-and-ready farm electric plant, supplying two farms with light, heat and power; and a Ward Leonard-type circuit-breaker for charging storage batteries] _Care of Battery_ Since specific directions are furnished with all storage batteries, it is not necessary to go into the details of their care here. Storage battery plants are usually shipped with all connections made, or plainly indicated. All that is necessary is to fill the batteries with the acid solution, according to directions, and start the engine. If the engine is fitted with a governor, and the switchboard is of the automatic type, all the care necessary in charging is to start the engine. In fact, many makes utilize the dynamo as a "self-starter" for the engine, so that all that is necessary to start charging is to throw a switch which starts the engine. When the battery is fully charged, the engine is stopped automatically. The "electrolyte" or solution in which the plates of the lead battery are immersed, is sulphuric acid, diluted with water in the proportion of one part of acid to five of water, by volume.