_Build your own batteries first_. Then after you have learned how they are made and something about their proper care buy them from some reliable electrical house. Batteries are always interesting to the average experimenter, and when properly made are one of the most useful pieces of apparatus around the home, laboratory, or shop that it is possible to construct. Many hundreds of thousands of experiments have been carried out by capable men in an effort to discover or devise a perfect battery, and the list of such cells is very great. Only the most common forms, which are simple and inexpensive to construct but will at the same time render fair service, have been chosen for description. Cells are usually considered _one_ element or jar of a battery. A _cell_ means only one, while a _battery_ is a _group_ of cells. It is not a proper use of the word to say "battery" when only _one_ cell is implied. This is a very common error. *The Voltaic cell* is called after its inventor, Volta, a professor in the University of Pavia, and dates back to about the year 1786. [Illustration: Fig. 52.—The Voltaic Cell.] A simple voltaic cell is easily made by placing some water mixed with a little sulphuric acid in a glass tumbler and immersing therein two clean strips, one of zinc and the other of copper. The strips must be kept separate from each other. The sulphuric acid must be diluted by mixing it with about ten times its volume of water. In mixing acid with water always remember never to pour water into acid but to perform the operation the other way and pour the acid into the water. A copper wire is fastened with a screw or by soldering to the top of each of the strips, and care must be exercised to keep the wires apart. As has been said, the zinc and copper must never be allowed to touch each other in the solution, but must be kept at opposite sides of the jar. The sulphuric acid solution attacks the zinc, causing it slowly to waste away and disappear. This action is called _oxidation_, and in reality is a very slow process of burning. The consumption of the zinc furnishes the electric energy, which in the case of this cell will be found to be sufficient to ring a bell or buzzer, or run a very small toy motor. As soon as the plates are immersed in the acid solution, bubbles will begin to rise from the zinc. These bubbles contain a gas called hydrogen and they indicate that a chemical action is taking place. The zinc is being dissolved and the _hydrogen_ gas is being set free from the acid. It will be noticed that no bubbles arise from the copper plate and that there is little if any chemical action there. In other words, it seems that the chemical action at one plate is stronger than that at the other.