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 ZnSO4, 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.
Here are three iron nails. I propose that you plate one with zinc and another with copper and then expose all three to the weather and see which will rust. I propose that you replate all the spoons at the cottage and the metal tops of the salt cellars with silver. Electro-plating results better if done slowly. Ten volts and .1 ampere will be sufficient current.
In the storage battery we generally use lead for both positive and negative plates and dilute sulphuric acid for the electrolyte. Hydrogen is liberated at the positive plate and oxygen unites with the negative plate. When the charging current is cut off the chemical action reverses, and an electric current is produced by the cell.
In all other batteries there is a destruction of one plate and of the electrolyte, which cannot be fully restored by a charging current, although in the case of the lead and sulphuric acid combination the charging and discharging of the cell may go on alternately for a very long period without permanent change or loss of any substance except water. There is, however, plenty of loss of energy in this as in other transformers. One hundred ampere hours of current expended to charge a storage battery will yield from seventy-five to eighty-five ampere hours while the battery is discharging.
The lead storage battery is, however, full of disappointments for those who do not properly care for it. It is irretrievably ruined if neglected and allowed to charge too far, or discharge too far, or evaporate too much water, etc. The voltage of a lead cell must not rise above 2.2 nor fall below 1.8. It must not be allowed to furnish at any one time a greater number of amperes than it is rated for. It must not stand idle too much. It must not be cleaned up and put away for a period. In fact, the lead-sulphuric acid battery is so poorly adapted to our need that I feel disposed to try Mr. Edison's new storage battery. This has nickel hydrate packed in tubes of metallic nickel for the positive plates and iron oxide pressed into pockets in a sheet of metallic iron for the negative plate. A solution of potassium hydrate in water is used for the electrolyte. This is said to be uninjured by being emptied out and left idle, as our batteries must be for a large part of the year. The e. m. f. of this battery is less than that of the lead battery, being only 1.2 volts. We shall therefore need ninety-six cells (type B-4) for the machine shop and ninety-one cells of the same kind for the cottage. Our dynamo will be unable to charge at one time more than sixty of these cells connected in series.