204 OUR PHYSICAL WORLD 



sized piece of window glass supported between the leaves of 

 two books so it is a little over a half-inch above the table. Rub 

 the upper surface of the glass briskly with a piece of silk or wool 

 cloth. Shortly the figures will dance as they fly up to the glass 

 on which the electricity is developed, become charged with it, so 

 fly away again to the table to which the charge is discharged, 

 when the process is repeated. 



This electricity that is developed by friction is known as 

 frictional electricity. You have probably heard it crackle while 

 combing your hair when it is dry and cool, or have felt and 

 seen the spark fly when, after shuffling across the rug, you have 

 presented your finger to some metal object like the radiator or 

 water pipe. The two kinds that are developed, one on glass 

 when it is rubbed with silk, the other on amber or sealing wax 

 when it is so rubbed, were at first called vitreous (glassy) elec- 

 tricity and resinous electricity. But later they were designated 

 positive and negative respectively, for when they come together 

 they neutralize each other and no charge is apparent. They 

 appear to be present in equal quantities in such substances and are 

 merely separated by rubbing. 



In 1749 Benjamin Franklin performed his famous kite experi- 

 ment. By this time men knew how to make quite powerful 

 frictional electric machines, so he knew from his work with these 

 that the electric spark has a zigzag course, crackles as it appears, 

 may set things on fire, can even kill small animals, and dis- 

 charges most readily from pointed conductors. He knew that 

 in many respects lightning behaved similarly, and so he surmised 

 that lightning was electricity discharging from cloud to cloud 

 or from a cloud to the earth, and that buildings might be pro- 

 tected from lightning stroke by setting in the ground near them 

 tall, pointed, metal rods in order that the electric discharge 

 would pass through them instead of through the buildings. This 

 seemed very absurd even to the scientists of his day, and his 

 suggestion was received only with amusement. But Franklin 

 was not to be easily discouraged. He decided to try an experi- 



