ELECTRICAL APPLIANCES AND MACHINES 249 



seeking and south-seeking poles of magnets where like 

 poles repel and unlike poles attract each other. The 

 electrifications which are imparted to glass by rub- 

 bing it with silk and to sealing wax by rubbing it with 

 flannel are opposite in the sense that an electrified body 

 that is attracted by one is repelled by the other. We 

 have, therefore, two kinds of electrification, and for 

 convenience we call one positive and the other negative. 

 Positive electricity is like that on glass when rubbed with 

 silk, and negative electricity like that on the sealing wax 

 when rubbed with flannel. We also have a law much 

 like that applied to magnets, namely, electrical charges 

 of like kind repel each other, while electrical charges of 

 unlike kind attract each other. 



172. Lightning Rods. Benjamin Franklin was the 

 first to prove that lightning and electricity are the same. 

 During a thunder storm he sent up a kite which had a 

 sharp-pointed wire on the top to receive or discharge 

 electricity. He used a hempen string which became a 

 conductor after it was wet. He held it with a silk hand- 

 kerchief, a non-conductor, so that he would not receive 

 a shock. From the metal key which was tied to the 

 hempen string near him, he was able to draw electric 

 sparks. This suggested to him that the electric charge 

 in the cloud could be neutralized by setting up a number 

 of sharp-pointed rods which would permit the induced 

 charge on the earth to escape into the air. The charged 

 particles of air would be drawn to the charge in the cloud 

 since they were of the opposite sign. 



Lightning rods made of good conducting material, 

 placed deep enough in the ground to reach damp earth 

 (for dry earth is a poor conductor), and provided with 

 good steel points on the top of the building, will neutralize 



