ELECTRICITY. 



ment of the cylinder has been obtained, 

 we may observe several other remark- 

 able phenomena, besides those of attrac- 

 tion. It' the experiment be performed 

 in a dark room, flashes of light, of a 

 bluish colour, will be perceived during 

 the friction, extending over every part of 

 the surface rubbed ; and sparks, attended 

 with a sharp snapping sound, will be 

 seen to dart around it in various direc- 

 tions. If a round body, as a metallic 

 ball, be presented to it, and moved from 

 one end to the other, a succession of 

 sparks will be obtained as the ball passes 

 along the surface ; and if the knuckle 

 be presented instead of the metallic 

 ball, each spark will be accompanied by 

 a pricking sensation. When the ex- 

 cited cylinder is brought near to the 

 face, an unpleasant sensation of tickling 

 is felt in the skin, as if it had been co- 

 vered with a cobweb. 



(11.) If a globe of metal be sus- 

 pended in the air by silk threads, and if, 

 while in this situation, it be rubbed by 

 an electric, such as silk, fur, or the 

 outside of the skin of a cat, it will also 

 become electrical, and exhibit the same 

 properties of attraction and of repulsion 

 as if it had been itself an electric. The 

 circumstance of its being thus insulated 

 or cut off from the contact of any sub- 

 stance, except the air and the electric 

 which sustains it, is essential to the suc- 

 cess of this experiment. 



(12.) Various modes have been de- 

 vised for exhibiting distinctly the attrac- 

 tive and repulsive agencies of electricity ; 

 and for obtaining indications of its pre- 

 sence, when it exists only in a feeble 

 degree. Instruments for this purpose 

 are termed Electroscopes. One of the 

 simplest of these is the Electroscope of 

 Haiiy, which is very similar to that 

 formerly proposed by Dr. Gilbert. It 

 consists of a light metallic needle, ter- 

 minated at each end by a light pith 

 ball, which is covered with gold leaf, 



Fig. 1. 



(13.) In some cases it is more con- 

 venient to employ a pair of similar 

 balls, suspended from a brass ball fixed 

 to the end of a glass handle, by very fine 

 silver wires, or by hempen threads, pre- 

 viously steeped in a solution of salt, and 

 afterwards dried. See fig. 2. 



Fig 2. 



(14.) C avallo has contrived an elec- 

 troscope of the same kind, which has 

 the advantage of being more portable 

 than that of Haiiy, while it is, perhaps, 

 equally sensible. It is formed by two 

 fine silver wires, each carrying at one 

 of their ends a little ball made of cork, 

 or of the pith of the elder tree ; the 

 other ends of the wires being suspended 

 from a cork, which is rather long, and 

 tapering at both ends, so as to fit either 

 way into the mouth of a varnished glass 

 tube, serving both as a handle to the 

 instrument when in use, and as a case 

 for it when carried in the pocket. When 

 it is to be employed as an electroscope, 

 the wires with pith balls are placed so 

 as to hang out from the end of the 

 tube, and will indicate by their diver- 

 gence any electricity which may be 

 communicated to them. (Fig. 4.) When 



Fig. 3. 



Fig. 4. 



and supported horizontally by a cap at 

 its centre, on a fine point. The attractive 

 or repulsive power of any electrified 

 body presented to one of the balls, will 

 be indicated by the movements of the 

 needle. 



the instrument is not in use, the wires 

 are put into the tube, by inverting it, 

 and closing it with the other end of the 

 cork. (Fig. 3.) 



(15.) For studying the circumstances 

 B 2 



