The familiar dry cell is not called a battery, but if two or more are connected together to form a source of electrical energy, we have a battery. The dry cell is known as a primary cell to distinguish it from the storage battery which is made up of secondary cells. The main difference between primary and secondary cells is that the secondary cells can be renewed by passing an electrical current through them--primary cells cannot be recharged by a current flow. When a dry cell is dead, it is thrown away. The principle of the so-called dry cell is practically the same as that used in the old-fashioned wet cell which was in common use 15 or 20 years ago as a producer of current for ringing door bells and for laboratory work.
TABLE NO. I
Material
Potential above zinc
-
Zinc
0
Volts
Iron
.36
“
Lead
.71
“
Tin
Hydrogen
.36
“
Copper
1.03
“
Silver
1.44
“
Gold
+
Carbon
1.74
“
The simplest of chemical cells is somewhat easier to study than the more complicated modern dry cell, so let us see what we can learn from a study of this old wet cell.
It consisted primarily of two different metals, zinc and copper (the electrodes) in an acid solution (the electrolyte) composed of one part of sulphuric acid in four parts of water. If two dissimilar metals are placed in diluted acid sultan, and the cell thus made is connected to an external circuit, electricity will flow through the circuit. The action of an electric cell is on of the most interesting of electrical and chemical phenomena. It starts when the sulphuric acid is poured into the water.
Volta, an Italian physicist, was the first man to discover that electrical pressure (e.m.f.) was developed between two unlike
