To obtain electricity from the dynamo we must spend money for the coal to make the steam which operates the steam-engine, or for the water which turns the water-wheel, as well as for an engineer in both cases. When we obtain electricity from a battery we must spend money for the chemicals and metals which are constantly consumed in the battery. PRIMARY BATTERIES An electrical battery is a device in which one or more chemical substances act upon a metal and a carbon, or upon two different metals, producing thereby a current of electricity, which will continue as long as there is any action of the chemicals upon the metal and carbon, or upon the two metals. Batteries for _producing_ electricity may be divided into two classes, called "open circuit" batteries and "closed circuit" batteries. Open-circuit batteries are those which are used where the electricity is _not_ required constantly without intermission--for instance, in telephones, electric bells, burglar alarms, gas-lighting, annunciators, etc. Closed-circuit batteries are those which are used where the effect produced must be continuous every moment, as, for instance, in electric lights and motors. The open-circuit battery is made in many different ways, so we only describe two of the principal ones. As we told you in an early part of this book, we do not know just what electricity is, nor why it is produced under the conditions existing in a battery. But we do know that by following certain processes and making certain chemical combinations we can make as much electricity and in such proportions as we want. The two metals, or the metal and carbon, in a battery are called the "elements," and to these are connected the wires which lead from the battery to the instruments to be worked by it. _The Leclanché Battery._--This form of open-circuit battery consists of a glass jar in which is placed the elements. One element consists of a rod of zinc, and the other element is carbon and powdered black oxide of manganese. These two (the carbon and black oxide of manganese) are placed in an earthenware vessel called a "porous cup." This is simply a small jar made of clay which is not glazed. Thus, the liquid which is in the glass jar penetrates through the porous cup to the carbon and manganese which it contains, and so the chemicals affect both these and the zinc at once, for, in order to obtain electricity, you will remember that the chemical action must take place at the same time upon both the elements in the same vessel. (Fig. 29.) The chemical substance used in this battery is sal-ammoniac, or salts of ammonia. A certain quantity of this salt is dissolved in water, and this solution is poured into the glass jar. When this is done the battery will generate electricity at once. [Illustration: Fig. 29] It should be remembered that the proper term for the chemical mixture which acts upon the elements in any battery is "electrolyte."