metals placed in contact with certain liquids (acids and alkalis). He discovered from experiment that all metals bear a certain relationship to one another, that various metals in a cell arrangement produce different voltages.
If you want to experiment, take a copper penny and a silver dime, and lay them on a piece of moist paper. If the two terminals of a pair of phones are placed one on the copper penny and the other on the silver dime, a click will be heard in the phones each time the contact is made or broken. This is proof that an electromotive force was created. If you substitute a zinc disc for the copper penny, a louder click will be heard showing that a larger e.m.f. was produced. From this we can also gather that the copper is more positive than the zinc.
Table No. I shows the different electrical relationships existing between metals.
Zinc is used as a standard, the others are considered as positive to zinc--thus iron is more positive than zinc, lead is more positive than iron and so on.
You will notice from this table that carbon is 1.74 volts positive above zinc--consequently, we might suppose that a chemical cell with zinc and a carbon electrode would deliver 1.74 volts. The fact of the matter is that primary hydrogen gas which is formed in a cell partially covers the carbon and this reduces the e.m.f. from 1.74 to 1.5 volts or 1½ volts. Zinc and copper according to the Table No. I should produce an e.m.f. of 1.03 volts, but this as you know can only be approximate.
A SIMPLE PRIMARY CELL
If we place in a glass jar a strip of zinc and a strip of copper or carbon and fill the jar with a weak solution of sulphuric acid (about 15 parts of water to one of chemically pure acid) and join by a metallic conductor the exposed terminals of these two strips we will find that the cell is capable of supplying a continuous flow of electricity along the conducting wire.
It will be observed as the current flows that the zinc strip wastes away in fact, the consumption of the zinc furnishes the electromotive force necessary to drive the current through the cell and through the external circuit. The chemical changes that take place within the cell, consisting of the zinc and copper strips immersed in the acid solution may be briefly described as follows: When the external circuit is complete and the current begins to flow, the acid attacks the surface of the zinc plate
