A dynamo is a very satisfactory source of current, provided some sort of power, such as a windmill, water motor or small engine is available for driving it. A hand dynamo is unsatisfactory for some purposes because the experimenter is usually unable to drive the dynamo and attend to other work at the same time. Batteries are the most expensive source of current but for many reasons, as explained above, are all that is available to the average experimenter. There are two classes of batteries, known as 1. Primary batteries; 2. Secondary or storage batteries. *Primary Batteries* are those which generate their own current by the action of some chemical such as, for example, an acid upon a metal. *Secondary Batteries* derive their current from a dynamo or other source of electricity and store it away in the form of *Chemical energy* until it is used up. A storage battery might be likened to a pail, which can be carried to a dynamo and filled full of electricity. Those who possess a storage battery can recharge it themselves from the 110-volt lighting or power circuit, from a dynamo or by taking it to an automobile garage where recharging is done. Homemade batteries are not as practical as those which can be purchased ready made, but the knowledge and experience gained in making your own are so valuable that every experimenter is urged to start in this way. Various materials such as zinc, copper, carbon, etc., can be used to make some very interesting and valuable batteries. One of the most common mistakes made in reference to cells and batters is in calling a single cell a *battery*. One cell is a cell. More than one cell connected together is a *battery of cells* or simply a *battery*. The Voltaic Cell. The first practical cell was invented in 1786 by an Italian professor named Volta and it is, therefore usually called the Voltaic cell. A Voltaic cell may be easily made by the experimenter, by placing some water, mixed with sulphuric acid, in a glass tumbler or a jelly jar and then immersing therein a strip of zinc and a strip of copper, each about four inches long and one inch wide. The strips must be kept separate from one another and should be scraped clean and bright before they are placed in the solution. A copper wire is fastened to the top of each one of the strips. The acid solution should be composed of one part of acid, mixed with ten parts of water. When mixing acid and water, always remember to pour the acid into the water and never pour water into acid. Otherwise the solution will suddenly become very hot and is liable to crack the jar. Acid should always be mixed in a glass or earthenware vessel and never in any sort of a wooden or metal receptacle, because it will attack and dissolve metals and wood.