ELECTRICITY. 



17 



which covers the cylinder, until it comes 

 to the immediate vicinity of the metallic 

 points, which being placed at a small 

 distance from the cylinder, absorb nearly 

 the whole of the electricity as it passes 

 near them, and transfer it to the prime 

 conductor. Positive electricity is thus 

 accumulated in the prime conductor, 

 while the conductor connected with the 

 cushion, being deprived of this electri- 

 city, is negatively electrified. 



But if both these conductors are 

 insulated, this action will soon have 

 reached its limit : for when the cushion 

 and its conductor have been exhausted 

 of their fluid to a certain degree, they 

 cannot by the same force of excitation 

 supply any further quantity to the glass. 

 In order to enable it to do so, we must 

 replenish it, as it were, that is, restore 

 to it a quantity equal to what it has 

 lost. This purpose will be answered by 

 placing it in communication with a con- 

 ducting body of large dimensions ; or, 

 what is still more effectual, by making 

 it communicate with the earth, which is 

 an inexhaustible source of electric fluid. 

 In order, therefore, to supply the prime 

 conductor with a constant stream of 

 electricity, we must destroy the insula- 

 tion of the cushion, by placing on the 

 conductor to which it is fixed, a metal- 

 lic chain, or wire, extending to the 

 ground. If, on the other hand, we wish 

 to obtain negative electricity, by means 

 of the same machine, we must keep the 

 negative conductor insulated, and con- 

 nect the prime conductor with the 

 ground, in order to allow the fluid to 

 escape from it as soon as it is collected 

 from the cylinder. The fluid will thus 

 continue to be drawn without interrup- 

 tion from the negative conductor, as it 

 now meets with no impediment to its 

 discharge on the opposite side of the 

 machine. 



That the quantity of positive electri- 

 city produced in one conductor is exactly 

 equal to that of the negative electricity 

 in the other, is proved by the fact that, 

 if the two conductors are connected by a 

 wire, no signs of electricity are obtained 

 in any of the conductors on turning the 

 machine : but if the wire be not con- 

 tinuous, but interrupted by short inter- 

 vals, a succession of sparks appear at 

 each interval, indicating the passage of 

 a stream of fluid from the one side to 

 the other ot the apparatus. 



(64.) A person standing on a stool 

 with glass legs is thereby insulated; 

 and if, in this situation, he touch the 



prime conductor, either with his hand, 

 or through the intermedium of a metal- 

 lic rod, or chain, he may be considered 

 as forming part of the same system of 

 conductors. When the machine is 

 worked, therefore, he will partake with 

 the conductor of its charge of electri- 

 city, and sparks may be drawn from 

 any part of his body by the knuckle of 

 any other person who is in communica- 

 tion with the ground. 



CHAPTER IV. 



Effects of Electrical Attraction and, 

 Repulsion. 



(65.) HAVING obtained, by the elec- 

 trical machine, the means of accumu- 

 lating considerable quantities of elec- 

 tricity, we are enabled to multiply 

 and extend our observations of the 

 phenomena, and to examine with 

 more precision their correspondence 

 with the results of theory. The effects 

 of electrical attractions and repul- 

 sions may be exhibited much more 

 distinctly, and on a larger scale than 

 with the simpler instruments we had 

 previously employed. The experiments 

 formerly mentioned on the alternate 

 approach and recession of light bodies, 

 may be repeated with either conductor 

 of the machine, when charged with 

 electricity, and we may note with more 

 accuracy the differences which occur in 

 the rapidity with which the changes 

 from one electrical state to another 

 take place according as the bodies are 

 more or less good conductors of elec- 

 tricity. A pith ball, or a fragment of 

 gold leaf, is very strongly and immedi- 

 ately attracted by the electrified con- 

 ductor, and the instant after it has come 

 into contact with it, is repelled ; but 

 it is now attracted by the other bodies 

 in its neighbourhood, to which it com- 

 municates its own electricity, and then 

 is again in a state to be influenced by 

 the conductor, and to be again attracted : 

 and this alternation of effects will con- 

 tinue as long as the conductor remains 

 charged. 



(66.) These alternate and rapid 

 movements are best seen by placing 

 these small bodies between two metallic 

 plates, placed as in fig. 11, the one over 

 the other, at a certain distance ; the 

 upper one communicating with the 

 prime conductor, the lower one with the 

 ground. If figures of men and women 

 are cut out of paper and placed between 



