ELECTRICITY. 



diictor, N P, will be left with an ex- 

 cess of negative electricity. 



" It will be obvious, from the above 

 statement, that when light bodies, es- 

 pecially if they be conductors, are at- 

 tracted by electrified surfaces in their 

 vicinity, they are thrown by induc- 

 tion into opposite electrical stales ; 

 and when the hand is brought near 

 the excited conductorof the electrical 

 machine, it becomes negative, and 

 remains so until the equilibrium is 

 restored by the passage of a spark ; 

 which phenomenon is supposed to be 

 the result of the combination of the 

 two electric fluids. 



" Many important phenomena of 

 BQ electrical accumulation are 

 Tl explained by reference to the 

 J^ principles of induction, and 

 among them the action of the 

 Leydcn jar, or vial. A thin 

 glass jar, or bottle. A, is coat- 

 "'*^^**^ ed inside and out, to within 

 three or four inches of its mouth, with 

 some conducting substance ; tin foil, 

 being especially convenient for the 

 purpose, is generally used ; and a 

 metallic rod, projecting a few inches 

 from the aperture, and surmounted 

 by a brass ball, B, communicates with 

 the interior coating. 



" When the ball is applied to the 

 prime conductor of the electrical ma- 

 chine, and the outer coating commu- 

 nicates with the ground, the interior 

 acquires a positive and the exterior 

 a negative charge ; and on making a 

 communication by means of a con- 

 ductor between the inner and outer 

 coatings, the electricities are annihi- 

 lated with the production of a bright 

 spark and explosion, and by a most 

 disagreeable sensation, called the 

 electric shock, if the body be made 

 part of the circuit. When several 

 jars are so arranged that their inte- 

 rior and exterior coatings are all sep- 

 arately connected, the assemblage 

 constitutes the electrical battery. 



'• In the common electrical machines, 

 electricity is excited by the friction of 

 the plate or cylinder of glass upon the 

 cushions or rubbers ; and the glass 

 becomes positive, and communicates 

 the same state to the opposed con- 



Y 



ductor, generally termed the prtme 

 conductor of the machine ; the rubber 

 becomes negative, and is sometimes 

 connected with a second conductor. 

 " The annexed figures represent 

 the two com- 

 mon forms of 

 the electrical 

 machine. The 

 first is the cijl- 

 inder machine, 

 commonly call- 

 ed Nairne's ma- 

 chine. B is the 

 glass cylinder, 

 which is made 

 to revolve upon its axis by the mul- 

 tiplying wheels, V C, the necessary 

 friction for the electric excitation be- 

 ing produced by the cushion and silk 

 flap, D. A A are the positive and 

 negative conductors : the latter, bear- 

 ing the cushion, is adjusted as to its 

 requisite pressure upon the cylinder 

 by the screw at E. The conductors 

 are respectively supported and insu- 

 lated by the glass pillars G G, which 

 should be coated with lac varnish ; 

 and the axis of the cylinder rests upon 

 the pillars H H, which are also of 

 glass. The second figure represents 

 the plate machine, usually termed 

 Guthbertson's machine, in which A 

 is the prime con- 

 ductor, borne by 

 a stout glass stem 

 which is attached 

 to the frame of the 

 machine. B B are 

 the upper and low- 

 er pairs of cush- 

 ions, by which, to- 

 gether with the 

 silk flaps, C C,the 

 necessary friction 

 is obtained. E is the disk of plate 

 glass, which is made to revolve upon 

 its axis by the winch F. In this ma- 

 chine, as the cushions or rubbers are 

 not insulated, the negative electrici- 

 ty cannot be separately accumulated 

 or exhibited, as in the cylinder ma- 

 chine. 



" There are many other and highly 

 important causes of electric excita- 

 tion than those above adverted to ; 



253 



