20 



ELECTRICITY. 



the web of the silken worm descends 

 nearly to the bottom of the jar, and 



Fig. 1 5. 



carries at its lower extremity a trans- 

 verse needle. This needle consists of 

 either a filament of gum-lac, or a silk 

 thread or piece of straw coated with 

 sealing-wax. At one end it is termi- 

 nated by a small pith-ball, and at the 

 other by a disc of varnished paper, act- 

 ing merely as a counterpoise to the 

 ball. The upper end of the silk fibre is 

 affixed to a kind of button having a 

 small index, and capable of being turned 

 round upon a circular plate divided into 

 degrees. One side of the jar is perfo- 

 rated to allow of the insertion of a short 

 horizontal bar, having a small metallic 

 sphere at each of its ends, the one being 

 in the inside and the other on the out- 

 side of the jar; and the former being 

 so situated as just to allow the ball of 

 the suspended needle to come in con- 

 tact with it in the course of its revolu- 

 tion. By turning the button, or the 

 index, the needle may be brought into 

 this, or any other required position with 

 regard to the ball. It is found by ex- 

 periment that the angle of torsion of 

 the silk fibre is, within a certain range 

 of distance, very nearly in the direct 

 ratio of the force which acts in pro- 

 ducing the torsion ; and therefore, it the 

 two balls be placed in contact by turning 

 the button, and then similarly electri- 

 fied, the distance to which they are 

 repelled by the angular motion of the 

 suspended ball, attbrds a measure of 

 the repulsive force exerted. In like 

 manner, the distance which the sus- 

 pended ball is made to move when it 

 is attracted by the fixed ball, when the 

 two have opposite electricities, gives 

 accurate measures of the attractive 

 forces. It was by the employment of this 

 apparatus, in a very elaborate series of 

 experiments, that Coulomb was enabled 



to establish very satisfactorily the exact 

 law of variation, both of the attractive 

 and repulsive forces, arising from elec- 

 tricity, with relation to the distance, 

 which we have already stated. 



CHAPTER V. 

 Distribution of Electricity. 



(75.) IT had long been observed, that 

 the quantity of electricity which bodies 

 are capable of receiving, does not follow 

 the proportion of their bulk, but depends 

 principally upon the extent of their sur- 

 face. It was found, for instance, that a 

 metallic conductor in the form of a globe, 

 or cylinder, contains just as much elec- 

 tricity when hollow, as it does when solid. 

 Hence it was evident that the electricity 

 resides altogether at the surface, or at 

 least does not extend equally through- 

 out the whole mass of the body. But 

 it was only by applying to the theory 

 all the refinements of mathematical in- 

 vestigation, that precise notions could 

 be formed of the exact distribution of 

 the electric fluid in bodies of different 

 shapes. The labours of Cavendish, 

 Coulomb, Poisson, and Ivory, have fur- 

 nished the means of determining this 

 problem in every case, however com- 

 plicated; and whenever a comparison 

 has been instituted between the results 

 of experiment and of theory, the most 

 perfect agreement has been found be- 

 tween them. Thus all the phenomena 

 of electricity are found to be in exact 

 conformity with the mechanical conse- 

 quences of the theory : they can be an- 

 ticipated with rigorous precision, and 

 can even be reduced to numerical cal- 

 culation in their minutest details, as 

 well as in their most intricate combina- 

 tions. 



(76.) For the purpose of measuring 

 the proportional quantities of electri- 

 city with which different parts of the 

 same; or of different bodies are charged, 

 no instrument is so well fitted as the 

 balance of Coulomb, of which an ac- 

 count has just been given. What pecu- 

 liarly adapts it for these experiments, is 

 its extreme sensibility, by which the 

 slightest variation in the intensity of the 

 attractive or repulsive force produces a 

 very considerable effect in the movement 

 of the horizontal needle. In some of 

 the experiments related by Coulomb, a 

 force only equal to the 279ih of a grain 

 was sufficient to make the needle per- 

 form an entire revolution round the 



