176 



THE POPULAE EDUCATOR 



A 



not diverge, showing that there is no free electricity present. 

 If, however, we place a metal ball, connected with the ground, 

 inside the cylinder, but without allowing it to touch it, induction 

 will be produced, and the presence of a small quantity of free 

 electricity on the inner surface will be shown. 



Various experiments have been tried, which prove most con- 

 clusively that, except in cases like the above, where induction 

 is specially produced, all the electricity exists on the outer 

 surface. A large metallic box was insulated and highly 

 charged, so that sparks 

 were freely given off from 

 its surface, and yet, when 

 a person with a delicate 

 electrometer got inside, no 

 effect at all was produced 

 on him or on the instru- 

 ment. So, too, a powerful 

 battery may be discharged 

 through a large case, inside 

 which a person is con- 

 cealed, without his feeling 

 it in any way. A simpler 

 experiment, proving the 

 same fact, is to mount a 

 cylindrical or egg-shaped 

 body on an insulating 

 stand, and provide two 

 hemispherical caps which, 

 together, just fit it. These 

 should be fixed to insula- 

 ting handles. If now we 

 charge the globe, and thec 



place the covers on it, and quickly remove them, we shall find 

 that all the electricity has left the globe and passed into the 

 covers. 



If we take a conical gauze net, and fix its open mouth 

 to an insulated ring, and also fir two silk strings to 

 its apex, so that by pulling one or other of these it may 

 be turned , inside out, we shall find, 

 on charging it, that the electri- 

 city will always exist alone upon 

 the outer surface, and no trace of 

 it will be found on the interior. 



Since, then, the distribution of 

 electricity on any surface is thus 

 affected by induction, wo should 

 expect to find that it is by no means 

 evenly distributed over any charged 

 surface ; and such is the case, a 

 larger portion being always collected 

 on that side which has the best con- 

 ducting surface opposed to it. Thus, 

 if two metallic globes be suspended 

 near together, and one of them be 

 charged, the greater portion of its 

 electricity will be attracted towards 

 the side which is nearest to the 

 other globe. 



In these experiments we want 

 some instrument which shall not 

 only manifest the presence of 

 electricity, but give us a better 

 idea of its intensity than can be 

 furnished by the electroscopes 



already described, and such an instrument is represented 

 in Fig. 5. It consists of a needle of shellac carrying a small 

 gilt disc, n, at one end. This is suspended by a loop, o, at the 

 end of a very fine wire, or sometimes by a piece of unspun silk, 

 and the degree to which this is twisted is the measure of the 

 electricity. The wire is fixed at the upper end to a graduated 

 circle, c, opposite one side of which is a pointer, a, which enables 

 the angle through which the circle is turned to be easily read 

 off. The needle is suspended in a glass case, round the inside 

 of which a graduated ring is marked. At r is an aperture by 

 which a metal ball, m, called the carrier ball, supported on a 

 glass rod, i, can be inserted. The zero point on the upper circle 

 is then brought opposite to the pointer a, and the tube d is 

 turned till the disc n is exactly opposite the ball. This ball 



Fig. 4. 



is now removed, and allowed to touch the substance or part of 

 the body whose electricity is to be determined. It thus acquires 

 a portion of its charge, and, being replaced through the aperture 

 r, it immediately attracts the disc n ; after contact, however, 

 it repels it with a force depending on the intensity of the charge, 

 and to measure this we turn the wire by means of the circle at 

 the top until the disc n is brought within a certain fixed 

 distance, say ten degrees, of m. The angle through which the 

 circle has to be turned in order to do this shows the intensity 



of the charge. 



As the vapour of the air 

 would rapidly carry away 

 the electricity, a small 

 saucer containing some 

 substance, such as chloride 

 of calcium, that powerfully 

 attracts the moisture, is 

 placed inside the glass 

 case, and serves to keep 

 the air within perfectly 

 dry. The apparatus is 

 known as Coulomb's Tor- 

 sion Electrometer. 



If we take charged cylin- 

 ders similar to that shown 

 in Fig. 3, and examine the 

 amount of electricity found 

 on different parts of their 

 surfaces, we shall find that 

 there is always a much 

 greater amount at the ends 

 than at the middle. The 



reason of this is, that, being more curved, they face a larger area, 

 and therefore are more powerfully acted upon by induction. If 

 the ends taper there will be a still greater accumulation, while if 

 they end in a sharp point the induction will be such that the 

 electricity will at once escape. Hence, in the manufacture 

 of electrical machines and apparatus great care must be 

 taken to avoid the presence of sharp 

 points or rough edges. This also 

 explains why we can obtain longer 

 sparks from the end of the con- 

 ductor of a machine than from its 

 sides. 



By means of the torsion electro- 

 mete, we may also investigate the 

 laws of the attraction and repulsion 

 of electrified bodies, the most impor- 

 tant of which is that the attraction 

 or repulsion varies inversely as the 

 square of the distance. Thus, if 

 two bodies be removed to double 

 the distance from one another, 

 their attraction will only be one- 

 fourth as great, at three times the 

 distance one-ninth as great, and 

 so on. 



An amusing experiment, that may 

 easily be tried, shows us more clearly 

 the principle of induction. Take a 

 sheet of glass, and having warmed 

 it well, support it at the sides on two 

 books, or blocks of wood, so that 

 it may be about an inch above the 

 table. Cut some figures out of stiff paper, or some balls of 

 elder pith, and lay them on the table underneath the glass. 

 On rubbing it now with a piece of silk or leather smeared with 

 amalgam, the pieces of paper will begin to jump rapidly up and 

 down between the table and the glass. An apparatus of this 

 kind is now made as a toy, and sold under the name of the 

 Merry Mountebanks. The explanation of these phenomena is 

 simple. When the upper surface of the glass is rubbed, electri- 

 city is evolved on it; this sets free a corresponding amount 

 on the other side of the glass, the light substances under it are 

 therefore attracted, and rise to touch it, but in so doing they 

 acquire a certain portion of its electricity, and are accordingly 

 repelled. On touching the table, however, they part with the 

 superfluous charge, and are therefore again attracted, as at first 



Fig. 5. 



