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CHAP, i.] ELECTRICITY AND MAGNETISM. 7 



5. Simultaneous production of both Electrical 



States. Neither kind of electrification is produced 

 alone ; there is always an equal quantity of both kinds 

 produced ; one kind appearing on the thing rubbed 

 and an equal amount of the other kind on the rubber. 

 The clearest proof that these amounts are eqital can be 

 given in some cases. For it is found that if both the 

 electricity of the rubber and the + electricity of the thing 

 rubbed be imparted to a third body, that third body will 

 show no electrification at all, the two equal and opposite 

 electrifications having exactly neutralised each other. 



In the following list the bodies are arranged in such 

 an order that if any two be rubbed together the one 

 which stands earlier in the series becomes positively 

 electrified, and the one that stands later negatively 

 electrified : Fitr, wool, resin, glass, silk, metals, sul- 

 phur, indiarubber, guttapercha, collodion. 



6. Theories of Electricity. Several theories have 

 been advanced to account for these phenomena, but all 

 are more or less unsatisfactory. Symmer proposed a 

 " two-fluid " theory, according to which there are two 

 imponderable electric fluids of opposite kinds, which 

 neutralise one another when they combine, and which 

 exist combined in equal quantities in all bodies until 

 their condition is disturbed by friction. A modification 

 of this theory was made by Franklin, who proposed 

 instead a "one-fluid" theory, according to which 

 there is a single electric fluid distributed usually uniformly 

 in all bodies, but which, when they are subjected to friction, 

 distributes itself unequally between the rubber and the 

 thing rubbed, one having more of the fluid, the other 

 less, than the average. Hence the terms positive and 

 negative, which are still retained ; that body which is 

 supposed to have an excess being said to be charged 

 with positive electricity (usually denoted by the plus sign 

 + ), while that which is supposed to have less is said to 



be charged with negative electricity (and is denoted by 



