Electricity 



205 



poles attract and like poles repel each other. There is also a 

 law quite similar to that in regard to magnets: Unlike electrical 

 charges attract and like charges repel each other. 



The Electroscope. The electroscope is an instrument for 

 finding out whether or not an object is electrically charged. It 

 usually consists of a metal rod passed through the stopper of 

 a glass jar and having a round piece of metal on the upper 

 end and a hook on the lower end. To this hook are attached 

 two equal-sized strips of gold leaf which hang below the cen- 

 ter of the jar. When an object is brought in contact with the 

 metal top, the strips of gold leaf will spread apart if the object 

 is charged, but will not move if it is not 

 charged. 



A simple electroscope may be made by sus- 

 pending a small pith ball from a standard by 

 means of a silk thread. The pith ball may then 

 be charged by touching it with a charged amber 

 rod. When the charged amber is brought near 

 it the second time, since it has the same kind of 

 a charge as the pith ball, it will repel the ball. 

 Two pith balls may be suspended and used like 

 the two gold leaves in the experiment above. 

 Franklin used two linen threads suspended in 

 this manner to detect charges of electricity. 



Conductors and Insulators. Substances 

 through which electric charges pass readily are 

 spoken of as conductors. Substances through 

 which electric charges will not pass are called insulators. Among 

 the good conductors are such metals as silver, copper, and alumi- 

 num. While pure water is a non-conductor, the addition of a 

 moderate amount of any acid or salt makes it a good conductor. 

 Among the best insulators are rubber, glass, porcelain, wood, and 

 dry air. You have probably noticed that iron and copper wire are 

 largely used in electrical devices and that glass and porcelain are 

 also used. Can you suggest reasons for all four ? 



Potential. Potential is an important term in electricity. 

 The potential of any point in an electrical circuit is the amount 



A SIMPLE ELECTRO- 

 SCOPE 



It shows whether or 

 not an object is elec- 

 trically charged. 



