At the surface of the other metal, which may be, and frequently is, copper, small bubbles of the gas called hydrogen are produced. This gas rises to the surface of the liquid and gradually passes off into the air. But its presence offers resistance to the passage of the current; so that generally there is associated with the copper a supply of the gas oxygen. Oxygen and hydrogen are always very eager to mix with each other, and, therefore, when the hydrogen bubbles appear they are quickly taken up by the oxygen near by. The mixture of these two gases forms water, which becomes part of the solution. All of this happens so quickly that the hydrogen cannot be perceived so long as there is any oxygen left in the copper-oxide plate. [Illustration: Fig. 34] In the Edison Primary Battery (Fig. 34) the plates are zinc, known as the negative, and copper oxide (copper and oxygen), or the positive. These are suspended in a solution of caustic soda and water, the plates and solution being contained in jars of glass or porcelain. The plates are provided with suitable wires for connecting the cells with one another and with the lamps, motors, or other devices which they are to operate. There are usually two zinc plates and one copper-oxide plate, or multiples thereof. The quantity of current that may be withdrawn depends on the size and number of the plates, as well as upon their construction and arrangement. The voltage of these cells is low, being about 0.65 volt each; but this is more than compensated for by the fact that the internal resistance of the battery is so low that the voltage is not perceptibly affected even at continuous high-discharge rates, and that the voltage remains practically constant throughout the life of the cell. Furthermore, when the battery is not in use there is practically no local action. Consequently, the cells may remain on open circuit (that is, doing no work) for years and there will be no loss of energy. The cell will then operate with the same practical efficiency as if it were new. In some classes of work this battery remains in service from four to six years without attention. Another peculiar advantage of this battery lies in the fact that the plates and the electrolyte are so well proportioned that they are all exhausted at the same time, and then new plates and solution can be put in the jar, restoring it to its original condition. These batteries are used in great numbers for railway signal work and for other purposes, such as fire and burglar alarm systems, various telephone functions, operation of electric self-winding and programme clock systems, small electric-motor work, for low candle-power electric lamps, gas-engine ignition, electro-plating, telegraph systems, chemical analysis, and other experimental work where batteries are required that will remain in use for long periods of time without requiring any attention or renewal.