ELECTRICITY. 



(3.) The luminous effects of electricity are, of 

 course, most visible in the dark. The light of the 

 spark is intense when the machine is powerful 

 and in good order. When short, it seems straight, 

 but it gets more and more irregular and zigzag 

 the longer it is. If the ball of a Winter's machine 

 be allowed to discharge into the air, the spark will 

 be seen to spread out into the form of a brush. 

 But if it be made to discharge silently from a 

 point, the point will be seen, in the dark, crowned 

 with a tuft of light, which is usually called the 

 glow. The glow is more readily got, and be- 

 comes much more extended in rarefied air. A 

 tall glass jar, from which the air can be pumped 

 out, is used to shew this. It has a brass ball at 

 each end, and the electric spark passes between 

 these by putting one end in connection with the 

 machine, and the other with the ground. The 

 glow throughout the jar has the appearance of a 

 column of mauve-coloured flame. But the colour 

 of the luminous discharge varies with the nature 

 of the metals between which it passes, and also 

 with the nature of the gaseous medium. The 

 flickering light thus produced has a striking 

 resemblance to the Aurora Borealis. 



ELECTROSCOPES AND ELECTROMETERS. 



Instruments for detecting the presence of feeble 

 electricity are called electroscopes; and those for 

 measuring the degree of excitement present in 

 any body are termed electrometers. Both are in- 

 dispensable in electrical inquiries. 



The simplest form of electroscope is the electric 

 pendulum already mentioned. 



Far more delicate, however, is what is known 

 as the gold-leaf electroscope. A 

 good form of this is shewn in 

 fig. 13. Two strips of gold- 

 leaf about hah an inch broad 

 are fixed to a brass rod so as 

 to hang inside a glass globe 

 of about four inches diameter. 

 The brass rod ends externally 

 in a ball, and is insulated with 

 sealing-wax from the brass fit- 

 ting of the neck. Before the 

 rod and leaves are finally fixed, 

 the interior of the globe is 

 thoroughly dried, that the in- 

 sulation of the leaves may be as 

 perfect as possible. When so finished, this forms 

 a very sensitive instrument : an extremely small 

 amount of electricity being sufficient to make the 

 leaves diverge. A strong charge would be very 

 apt to tear the gold-leaves, and it must therefore 

 be used with care. 



When we simply wish to test the kind of elec- 

 tricity on a body, induction is generally employed, 

 and in this way. The electrified body under 

 examination is brought near the electroscope, and 

 the knob of the instrument touched in presence of 

 the body. By withdrawing the electrified body, 

 we leave the leaves charged oppositely to itself, and 

 so they diverge. To find if they are positively or 

 negatively charged, we rub a glass rod and bring it 

 near the knob : it positively, the leaves will diverge 

 still more under the induction of the glass ; but if 

 negatively, they will collapse by the negative being 

 attracted to the positive of the glass rod. 



Fig. 14. 



Of electrometers, one of the first and simplest is 

 that shewn in fig. 14, and called the 

 quadrant pith electrometer. It con- 

 sists of a conducting-rod of brass 

 or of box-wood, with a divided semi- 

 circle of cardboard attached. A 

 straw ending in a pith-ball moves 

 about a pivot at the centre of the 

 semicircle, and indicates, by the ex- 

 tent of divergence from the brass 

 rod, the degree of electrification of 

 the instrument. It is very far from 

 delicate, however, and gives but a 

 rough test of electric tension. Its 

 chief use is to put on the prime 

 conductor of a machine, to give an 

 idea of its condition. 



A much more delicate instrument of this class, 

 and one which has had a considerable effect in 

 rendering electricity an accurate science, is Cou- 

 lombs torsion electrometer. It is represented in 

 the figure, and merits a short description here 

 (fig. 15). A light rod of 

 shellac, carrying a small 

 disc of gilt paper at one 

 end, is suspended by a silk 

 fibre, or fine silver wire, 

 within a glass cylinder A, 

 and a glass tube, B, sur- 

 mounting it The cylinder 

 is divided into degrees by a 

 graduated strip of paper 

 pasted round it ; and the 

 position of the gilt disc is 

 known by reference to this 

 graduation. Through an 

 opening in the lid of the 

 cylinder, a brass ball, at the 

 end of a shellac rod, can be 

 passed so as to be supported 

 on a level with the gilt disc. 



Before using the instrument, we must adjust it 

 so that, when the fibre hangs free from twist, the 

 gilt disc just touches the ball. If this ball be 

 electrified, it electrifies, and then repels the disc, 

 to a distance depending on the strength of charge 

 in the ball. Now, the principle on which its use 

 depends is that the force required to keep a fine 

 wire or fibre twisted through any number of turns 

 and parts of a tttrn, is just proportional to that 

 number of turns and parts of a turn. By turning 

 the button, to which the fibre is fixed at the top of 

 the tube, we can bring the disc to any distance 

 from the ball that we please, say to 30, as read off 

 on the cylinder. Then the number of turns and 

 parts of a turn (read off by reference to a gradu- 

 ated ring on the top of the tube), added to the 30 

 between the ball and the disc, gives the whole 

 amount of twist of the fibre from the first position 

 of no twist. This force of twist is just balanced 

 by the repulsion of the electricities in the ball and 

 disc, and is therefore a measure of that repulsion. 

 By a great number of such experiments, Coulomb 

 established the following laws of electrical attrac- 

 tion and repulsion : 



(i.) The force of attraction or repulsion between 

 unlike or like electricities decreases as the distance 

 between them increases, and decreases as the 

 square of the rate of increase of the distance. 

 Thus, the force of repulsion between two similarly 

 electrified piths, i inch apart, would be reduced 



Fig. 15- 



