constituent, to form again sulphuric acid as originally. [Illustration: Fig. 151] [Illustration: Fig. 152] We shall thus restore the conditions which prevailed in the first case as represented in Fig. 151. H_{2}SO_{4} is the chemist's designation of sulphuric acid and ZnSO_{4} is his expression for zinc sulphate. The experiment illustrates a storage battery so called. It might better be called a chemical transformer. It is wholly unnecessary that one rod be composed of zinc. If we begin with both rods of carbon immersed in a solution of ZnSO_{4}, and send into this cell the dynamo current, the carbon which acts as the negative pole will be coated with zinc in a short time, and we shall have in effect a rod of zinc and one of carbon as before. After a minute or two we may disconnect the generator and substitute in its place a bell as indicator, and it will ring, showing that we have transformed electrical energy into chemical energy which is now being retransformed into electrical energy. We say that we store electricity by this means, which is, however, no more true than that a farmer stores his farm in the bank when he sells it and deposits the money until he shall need it to buy another farm. Here is a very beautiful blue salt. I will drop a few crystals of it into a tumbler of water and dip in two carbon pencils connected to the dynamo current, using between fifty and sixty ohms of resistance in the circuit so as to have two amperes flowing. After a minute or two I lift out the negative carbon and you see that it is well plated with copper. The blue salt is copper sulphate. If we weigh the negative carbon, both before and after the experiment, we shall find that copper has been depositing at the rate of one ounce in twelve hours. If we reduce the current one half, making it one ampere, it will deposit copper at the rate of one ounce in twenty-four hours. One ampere will separate three ounces of lead in a day from a solution of any lead salt; it will separate .9 ounce of iron in a day from a solution of any iron salt, and it will liberate from water, which is a compound of hydrogen, one gallon of the gas in ten hours. The amount of chemical action is a measure of the amount of electrical energy expended. Before the present form of commercial wattmeter was devised electrolytic cells were used to determine what the consumer's bill for electricity should be each month. These chemical meters contained a solution of zinc sulphate for the electrolyte and both the positive and the negative plates were of zinc. While the current is passing, zinc from the solution is coated upon the negative plate and zinc from the positive plate takes its place in the solution, thus maintaining a constant strength of solution.