308 ELECTRICITY 



be employed in the construction of an electric cell; for ex- 

 ample, current may be obtained from a cell made with a zinc 

 plate and a platinum plate, or from a cell made with a lead 

 plate and a copper plate. Then, too, some other chemical, 

 such as bichromate of potassium, or ammonium chloride, may 

 be used instead of dilute sulphuric acid. 



Almost any two different substances 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 door bell. What is wanted 

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

 to the substances which will give this result. Zinc and lead 

 can be used, but the current resulting is weak and feeble, and 

 / for general use zinc and carbon are the most satisfactory. 



283. Electrical Terms. The plates or strips used in mak- 

 ing an electric cell are called electrodes ; the zinc is called the 

 negative electrode ( ), and the carbon the positive elec- 

 trode ( + ) ; the current is considered to flow through the wire 

 from the -f- to the electrode. As a rule, each electrode 

 has attached to it a binding post to which wires can be 

 quickly fastened. 



The power that causes the current is called the electro- 

 motive force, and the value of the electromotive force, gen- 

 erally written E. M. F., of a cell depends upon the materials 

 used. 



When the cell consists of copper, zinc, and dilute sulphuric 

 acid, the electromotive force has a definite value which is 

 always the same no matter what the size or shape of the 

 cell. But the E. M. F. has a decidedly different value in a 

 cell composed of iron, copper, and chromic acid. Each com- 

 bination of material has its own specific electromotive force. 



284. The Disadvantage of a Simple Cell. When the poles 

 of a simple voltaic cell are connected by a wire, the current 



