Fig. 54.—A Home-Made Voltaic Cell.
An ordinary tumbler or jelly glass will make a good battery jar. The exciting liquid should be composed of
One part of sulphuric acid
Ten parts of water
One of the disadvantages of the voltaic cell is that it becomes polarized, that is, small bubbles of hydrogen which are liberated by the chemical action collect on the copper plate and cause the strength of the battery to fall off rapidly.
There are a great number of elements, as the zinc and copper are called, and an even greater number of different solutions or excitants which can be employed in place of sulphuric acid to make a cell, forming an almost endless number of possible combinations.
Leclanche Cell. One of the most common forms of cell employed for bell-ringing, telephones, etc., is called the Leclanche cell, after its inventor, and consists of two elements, one of zinc and the other of carbon, immersed in a solution of sal ammoniac or ammonium chloride. This cell has an E. M. F. of 1.4 volts, which is about half as much again as the voltaic cell.
Fig. 55.—Carbon-Cylinder Cell, and Cylinder.
The most common form of Leclanche cell is illustrated in Figure 55. This type is usually known as a "carbon cylinder" cell because the positive element is a hollow carbon cylinder. The zinc is in the form of a rod passing through a porcelain bushing set in the center of the carbon cylinder. A battery of such cells can only be used successfully for open circuit work. The "open circuit" is used for bells, burglar alarms, telephone circuits, etc., or wherever the circuit is such that it is "open" most of the time and current is only drawn occasionally and then only for short periods.