200 GENERAL SCIENCE 



QUESTIONS 



1. Name some uses of insulators. Of conductors. 



2. How may a metal rod be electrified? 



3. Why do we connect the outer coating of a Leyden jar to the 

 earth? 



4. How do lightning rods protect buildings? Why are light- 

 ning rods pointed? 



'5. Explain how the " electrophorus " is charged. 



6. What do we mean when we say lightning " strikes "? 



7. Experiments with frictional electricity are performed more 

 easily on dry days. Why? 



Current Electricity. If a wire is connected with the 

 outer coating of a charged Leyden jar and then brought 

 near the knob at the top, a spark will pass and the two 

 coatings will be brought to the same electrical potential. 

 A current passes through the wire, but only for a very 

 short time. About the beginning of the 

 nineteenth century two men, Galvani (1786) 

 and Volta (1792), working independently, 

 discovered and studied a method of main- 

 taining a constant difference of electrical 

 potential between two substances and thus 

 a means of producing a continuous electric 

 current. If two different metals such as 

 copper and zinc are placed in dilute sul- 

 FIG. 170. A Sim- phuric acid and connected by a wire (Figure 

 pie Voltaic CeU. J^Q^ a con tinuous current of electricity 

 will pass from the copper through the wire to the zinc. 

 Such an arrangement is called a voltaic cell. In com- 

 mercial cells carbon is commonly used instead of copper. 

 Kinds of Cells. - - There are a number of different 

 kinds of cells, but in every case they depend upon chemi- 

 cal action of some sort. In the voltaic cell, zinc is slowly 

 decomposed with the formation of hydrogen and zinc 



