136 



MAGNETS AND ELECTRICITY 



4. Why is it dangerous to stand under a tree in a thunderstorm? 



5. Read about the life of Benjamin Franklin. What did he do for 

 science, for business, and for the American Colonies? 



150. Electric Currents. Magnetism and frictional 

 electricity, the two kinds of electrical phenomena already 

 studied, are closely related to the third form, electric 

 currents. The study of currents begins with the electric, 

 or " voltaic, " cell, a contrivance by which the energy set 

 free in certain chemical reactions appears in the form of 

 electricity, instead of as heat or light (cf. 51, 105, and 

 109). Methods of producing currents were first studied 

 by Galvani, in 1786, and by Volta, in 1792. 



We have already learned (cf. 103) that zinc and dilute 

 sulphuric acid give hydrogen. The chemical change also 



produces a substance called zinc 

 sulphate. Besides both of those the 

 reaction produces heat. But when 

 both zinc and copper are put into 

 dilute sulphuric acid (Fig. 125), 

 and the ends of the metals outside 

 of the liquid are joined, a current 

 flows through the wire, the metals, 

 and the liquid. The metals may 

 be joined by simply allowing them to touch each other. 

 The zinc wastes away, just as when it is used alone; but 

 instead of heat we get the current. 



A simple way to find out whether or not a current is passing through 

 the wire connecting the metals is to "taste" the current. We do this 

 by touching the tongue with the ends of the wires a little distance 

 apart. The current produces a sharp sensation. 



Coppe 



Cop- Zinc 

 per 



FIG. 125. 



Two Forms of the Simple Cell. 

 The current flows in the di- 

 rection of the arrows. 



