ELECTRICITY AND MAGNETISM. 687 



we have upon the screen a view of his instrument, which seems rude enough, in 

 comparison with the elaborate apparatus of our times. The first great work on 

 electricity and magnetism was the " De Magnete" of Gilbert, physician of Queen 

 Elizabeth, published in 1600. Galileo, already famous in Europe, recognized in 

 the methods of investigation used by Gilbert the ones which he had found so fruit- 

 ful and wrote of him, " I extremely praise, admire and envy this author." 



Gilbert made many interesting contributions to magnetism, which we shall 

 notice in another lecture, and he also found that sulphur, glass, wax and other 

 bodies share with amber the property of being electrified by friction. He con- 

 cluded that many bodies could not be thus electrified. Gray, however, found 

 in 1729 that these bodies were conductors of electricity, and his discoveries and 

 experiments were explained and described to the president of the Royal Society 

 while on his death bed, and only a few hours before his death. If precautions 

 are taken to properly insulate conductors, all bodies which differ in any way, 

 either in structure, in smoothness of surface, or even in temperature, are appar- 

 ently electrified by friction. In all cases the friction also produces heat, and if 

 the bodies rubbed are exactly aUke, heat only is produced. 



An electrified body will attract all light bodies. This gutta percha when 

 rubbed with a cat's skin attracts these bits of paper, and this pith-ball, and this 

 copper ball; it moves this long lath balanced on its centre, and deflects this ver- 

 tical jet of water into a beautiful curve. 



If a conductor is to be electrified, it must be supported by bad conductors. 

 This brass cylinder standing on a glass column has become electrified by friction 

 with the cat's-skin. My assistant will stand upon this insulating stool, and by 

 stroking his hand you will observe that with his other hand he can attract this 

 suspended rod of wood, and you will hear a feeble spark when I apply my 

 knuckle to his. 



DuFay, of Paris, discovered what he called two kinds of electricity. He 

 found that a glass rod rubbed with silk, will repel another glass rod similarly 

 rubbed, but that the silk would attract a rubbed glass rod. We express the facts 

 in the well-known law that like electricities repel each other, and unlike attract. 

 For a long time the nature of the distinctions between the two electricities was 

 not understood. It was found later that when the two bodies are rubbed to- 

 gether they become oppositely electrified, and that the two electricities are always 

 generated in equal quantity; so that if the two bodies are held in contact after 

 the rubbing has ceased the two electricities come together again and the electrical 

 phenomena disappear. They have been added together and the result is zero. 

 Franklin proposed to call these electricities positive and negative. These names 

 are well chosen, but we do not know any reason why one should be called posi- 

 tive rather than the other. The electricity generated on glass when rubbed with 

 silk is called positive. 



Let us now examine the distinction between positive and negative electrici- 

 ties somewhat more closely, aiding ourselves by two cases which are somewhat 

 anala^ous. 



