For rapid charging, when a battery has to be charged in four hours, the current should vary about as follows:
| 40 | per | cent. | of | total | 1st | hour |
| 25 | " | " | " | " | 2nd | " |
| 20 | " | " | " | " | 3rd | " |
| 15 | " | " | " | " | 4th | " |
For quick charging in three hours the rates should be: 50 per cent. 1st hour; 331/3 per cent. 2nd hour; 162/3 per cent. 3rd hour.
Mercury Arc Rectifier.—This is a device for obtaining direct current from alternating current for use in charging storage batteries. The transformation is obtained at a low cost, because the regulation is obtained from the alternating side of the rectifier, while the current comes from the direct current side.
Figs. 1,132 to 1,134.—Mercury arc rectifier outfit, or charging set. The cuts show front, rear, and side views of the rectifier, illustrating the arrangement on a panel, of the rectifier tube with its connection and operating devices.
The theory is as follows: In an exhaust tube having one or more mercury electrodes, ionized vapor is supplied by the negative electrode or cathode, when the latter is in a state of "excitation." This condition of excitation can be kept up only as long as there is current flowing toward the negative electrode.
If the direction of the voltage be reversed, so that the formerly negative electrode is now positive, the current ceases to flow, since in order to flow in the opposite direction it would require the formation of a new negative electrode, which can be accomplished only by special means. Therefore, the current is always flowing toward one electrode—the cathode, which is kept excited by the current itself. Such a tube would cease to operate on alternating current voltage after half a cycle if some means were not provided to maintain a flow of current continuously towards the negative electrode.