MAGNETISM AND ELECTRICITY 203 



discovered that sulphur, sealing wax, alum, and many other 

 substances behave in the same way when rubbed on cloth, 

 and he published the first book about electric phenomena in 

 1600 A.D., though he called such phenomena magnetic not electric. 

 There were thus many centuries during which nothing had been 

 added to the simple knowledge of the ancients in regard to 

 electricity. Then Otto Guericke, the man who made the first 

 air pump and who tried the famous experiment with the hemi- 

 spheres at Magdeburg (p. in) to show how great is air pressure, 

 discovered that electrified bodies may repel each other as well 

 as attract. You can easily repeat his experiment. Hang up a 

 pith ball or even a small round wad of tissue paper by a fine silk 

 thread. The results will be more emphatic if the ball is covered 

 with lightweight tin foil. Rub a glass rod or tube or a stick of 

 sealing wax with a piece of silk or wool cloth and bring the rod 

 near the ball. The ball promptly flies to the rod, adheres to it a 

 few moments until its surface also is charged with electricity- 

 like that on the rod, and then it flies away from the rod. Rub 

 again and present the rod to the ball, and now the ball is strongly 

 repelled, for both are charged with the same kind of electricity. 

 So Guericke said that a body charged with electricity draws to 

 itself one that is not charged but repels it the moment it is also 

 charged. 



It was not until 1762 that DuFay discovered that there were 

 apparently two kinds of electricity. When the ball is charged 

 with electricity from the glass rod rubbed with silk, it is repelled 

 by the glass rod, but if there be then presented to it a stick of 

 sealing wax rubbed with silk it is strongly attracted. Furthermore, 

 while it is repelled by the glass it is attracted to the surface of the 

 silk that has been used to rub the glass. So DuFay said there 

 are two sorts of electricity. Unlike kinds of electricity attract 

 each other but like sorts repel. An amusing method of demon- 

 strating the attraction and repulsion is as follows. Cut some 

 tiny dolls or figures of animals a half-inch high out of tissue 

 paper. Scatter these on a table so they will lie under a good- 



