Page:Electricity (1912) Kapp.djvu/57

Rh issues from the cell at the platinum or carbon plate, the terminal in connection with this plate is called the positive pole of the cell, the zinc terminal being the negative pole.

One of the most largely used types of cell with zinc-carbon electrodes is that designed by Leclanché. The liquid used in this cell is a dilute solution of salammonia, and the polarisation of the carbon is counteracted by the employment of a metallic oxide in contact with it. The carbon plate is placed into a porous pot and packed round tightly with a mixture of granular gas coke and manganese peroxide. This substance is a powerful oxydising agent; it gets hold of the molecules of hydrogen on their way to the carbon electrode, and thus prevents them settling there and causing a back e.m.f. of polarisation. This chemical action can, however, only go on at a moderate rate, so that the Leclanché cell is mostly used where weak and intermittent currents are required, as for instance, in the working of electric bells. If the cell is worked too hard, the chemical action, whereby polarisation is rendered innocuous, cannot keep pace with the rate at which hydrogen is carried to the carbon plate, and the e.m.f. of the cell, which under normal