DRY CELLS 



209 



cal, such as bichromate of potassium, or ammonium chloride, 

 may be used instead of dilute sulphuric acid. 



There are many different substances that will, under proper 

 conditions, give a current, but the strength of the current is 

 in some cases so weak as to be worthless for practical use, 

 such as telephoning, or ringing a doorbell. What is wanted 

 is a strong, steady current, and our choice of materials is limited 

 to the substances which give this result. Zinc and lead can be 

 used, but the current resulting is weak and feeble ; for general 

 use zinc and carbon are the most satisfactory. 



Electrical terms. The plates or strips used in making an 

 electric cell are called electrodes ; the zinc is called the negative 

 electrode ( ), and the carbon the positive electrode (-f ) ; the 

 current is considered to flow through the wire from the positive 

 to the negative electrode. As a rule, each electrode has & 

 binding post to which wires can be quickly fastened. 



The force that causes the current is called voltage or electro- 

 motive force ; the value of the voltage depends upon the mate- 

 rials used in the cell. When the cell consists of copper, zinc, 

 and dilute sulphuric acid, the voltage has a definite value which 

 is always the same no matter what the size or shape of the cell. 

 But the voltage has a decidely differ- 

 ent value in a cell composed of iron, 

 copper, and chromic acid. We need 

 a unit of voltage and of current just 

 as we need units of time, length, and 

 weight. The unit of voltage is 

 called the volt. The unit of current 

 is called the ampere. 



Dry cells. The simple voltaic 

 cell is not convenient for general use. 

 The dry cell is the most popular 

 modern cell because of the ease with FIG. 74. A dry cell. 



