ELECTRICITY. 



Faraday shewed that the air is the medium by 

 which the electric force passes between an inducing 

 and an induced body. He found that, if he 

 changed this medium and put shellac for air, the 

 amount of electricity induced was about twice as 

 much as with air. Hence he called shellac a 

 better dielectric, or medium of induction, than air. 

 Bodies, he would say, differ merely in the facility 

 with which their molecules communicate the elec- 

 tric state. If induction, then, be an action at a 

 distance, it is at no greater distance than between 

 the molecules of bodies. 



Distribution of Electricity. Whatever may be 

 the nature of electric force, it may be assumed as 

 self-evident that its intensity, or tension at any 

 part of the surface of a body, will be less in pro- 

 portion as the surface is increased. If two in- 

 sulated balls, one electrified and the other neutral, 

 be made to touch, the charge will be just divided 

 between them in proportion to their size. The 

 intensity of the excitement has nothing to do with 

 its quantity; and the same quantity may, in differ- 

 ent circumstances, possess very different inten- 

 sities. 



Not only does the size, but even the shape of a 

 body very materially influence the intensity of the 

 excitement at different parts of its 

 surface. A .ball, since every part 

 is alike, will have the same intensity 

 all over its surface. But an electri- 

 fied cylinder will have the greatest 

 intensities at its ends. With a body 

 such as that in fig. 8, the tension 

 will be far greater at the ends than 

 at the middle. In general, the 

 Fig. 8. farther a body is from the spheri- 

 cal shape, the farther will its 

 intensity be from uniform distribution. When the 

 body ends with a sharp point, the electricity seems 

 all to accumulate at the point, so that it is too 

 great to be retained on the body, and it bursts 

 from it instantly. 



ELECTRIC MACHINES. 



The ultimate object of all electrical machines is 

 the conversion of mechanical into electrical force. 

 We have, in the glass tube and silk rubber, the 

 embryo of such a contrivance. The mere form 

 which may be given to it for convenience does not 



affect the principle. 

 Cylinder Ma- 

 chine. Fig. 9 rep- 

 resents one of 

 this class of ma- 

 chines in its very 

 simplest form. 

 'It consists of a 

 strong glass cylin- 

 der, A, which can 

 be turned by a 

 handle. Against 

 one side of it 

 presses the rubber, 

 * E, which is a 

 cushion of leather 

 Fig. 9. stuffed with horse- 



hair, and having 



its pressure regulated by means of screws. Facing 

 the opposite side is a row of brass points, near to, 



yet not touching the glass. These metal points 

 communicate with a brass cylinder, G (rounded 

 at the ends, and mounted on well insulating 

 glass supports), which is called the prime con- 

 ductor. 



When the handle is turned, the friction of the 

 rubber excites positive electricity on the glass, and 

 negative on the rubber itself. Being a non-con- 

 ductor, the glass retains its electricity till brought 

 opposite the points, when it is discharged or picked 

 up, as it were, by the action of these. Were a 

 metal cylinder used in place of a glass, the 

 electricities would combine at the rubber as fast 

 as they were produced, and the mechanical energy 

 exerted in turning the cylinder would appear as 

 heat, instead of being transformed into electricity. 

 To prevent dissipation of the electricity from 

 the glass as it rushes through the air, a flap of 

 silk lies over the cylinder from the rubber to near 

 the metal points. In this way, then, positive 

 electricity collects on the prime conductor, and 

 negative on the rubber. If the latter were in- 

 sulated as well as the former, the tendency of 

 the electricities to unite at the rubber would 

 increase with its charge, and a limit to the 

 power of friction to prevent this would very soon 

 be reached. By connecting the rubber with the 

 ground, we get rid of the negative electricity, and 

 can thus go on accumulating to a much greater 

 degree the positive. In many instruments, the 

 rubber is connected with an insulated cylinder 

 similar to that of the prime conductor. Positive 

 or negative electricity may then be had at pleasure, 

 by connecting with a metal rod or chain either 

 the rubber or the prime conductor with the 

 ground. But except for this choice which it puts 

 in our power, it is not necessary to insulate the 

 rubber at all. 



Plate Machine. One of the best existing forms 

 of plate machines is that shewn in the figure, and 

 known as Winter's machine (fig. 10). A round 



Fig. 10. 



disc of stout plate-glass forms the excitable sur- 

 face. It is completely insulated by the glass sup- 

 port e, and the glass axle a, which turns it The 

 chief advantage of the plate arrangement is that 

 both sides of the glass can be rubbed at once. In 

 this machine, the rubbers are triangular-shaped 

 pieces of wood, with a padding of one or two 



