To study the simple cell
A plate of copper and a plate of zinc, each fitted with a terminal, are dipped into a beaker containing very dilute sulphuric acid, and both plates are carefully examined. So long as the plates are not allowed to touch, no action will be observed at the copper plate. Owing to local action, however, hydrogen bubbles will generally be observed on the zinc. If this is the case the zinc should be taken out, amalgamated with mercury as described in the last paragraph and then returned to the cell.
When the terminals of the cell are joined by a length of copper wire hydrogen bubbles begin to form on the surface of the copper. The same thing happens if the plates are allowed to touch either outside or inside the acid. The electromotive force of the cell may be measured by connecting a voltmeter to the cell terminals, but before taking a reading the copper plate should be wiped clear
of hydrogen bubbles. The e.m.f. of a simple cell is slightly less than I V. If a small pocket-lamp bulb is connected to the terminals of the cell its filament will glow dully for a few moments and then go out. Once more, hydrogen bubbles will be seen on the copper plate, and if a voltmeter reading is taken the terminal potential difference will be seen to have fallen to about 0.2 V.
The original e.m.f of the cell may be restored by removing the hydrogen bubbles from the copper plate by vigorous brushing with a small paint brush, but it is much better to add a depolarizing agent to the cell. Potassium dichromate is suitable for this purpose, and it acts by oxidizing the hydrogen to form water. When a little potassium dichromate solution is added to the acid the hydrogen bubbles rapidly disappear and the original e.m.f. is restored. At the same time the lamp begins to glow continuously.