no 



i MM i.vnu\ run, 



T1O3. 



i 



necessary to wipe them when in use, a* they often gather 

 dost on their surface* by the electrical attraction which 

 luoed a* they are charged. 



It u often a matter of surprise to obnerve the quantity 

 of .lint which collect* on a charged surface ; the ell- 

 which U to much diminish the power of both the machine 

 and battery. 



A very convenient form of dwcharger for powerful bat- 

 tarie* U reprmonUol in Fig. 23. It ha* the advantage 

 of discharging the bat- ( . jj 



tory at any period, or 

 at any amount of charge 

 the experimenter may 



two upright glacs rod.H, 

 a and 6. On a, a bran 

 knob and hook are 

 fixed, from which ex- 

 tend* a wire to the out- 

 aide of the battery, or 

 to any intervening 

 piece of apparatus, and 

 thence to the outside 

 of the battery, e e is a 

 rod of brass, moving readily on a pivot, d ; and g is a 

 sliding weight, by means of which the balance between 

 the knobs, e and e, can be adjusted. The knob, e, tends 

 to raise t upwards ; but by shifting g from d to e, the 

 amount of charge in the battery can be regulated. The 

 inside of the battery is attached by a wire to the book on 

 the knob /. The engraving represents the arrangement 

 as if a battery were being charged. When it is fully so, 

 the knob, r, will fall down towards that on the rod, a, and 

 the discharge will at once take place. We need scarcely 

 Btate, that the principle on which this arrangement 

 works, is that of electrical attraction between the 

 charged knob, e, and that fixed on a. Besides the con- 

 venient character of this apparatus, it prevents the un- 

 pleasant and, possibly, dangerous effect which might 

 arise when a large battery is discharged by moans of the 

 discharger previously described.* 



It is, however, occasionally necessary to use a hand 

 discharger ; and the handle for that purpose should be a 

 glass rod, not less than two feet in length, so as to pre- 

 risk of accident. 



We know of no better mode of regulating an electric 

 charge in a battery, than tliat wo have just dcscriln-d, 

 aud which we always prefer to use. Many others have 

 been proposed, but as they present no special advantage, 

 we shall refrain from describing them. 



To assist those who may not be able to purchase ex- 

 tensive apparatus, we shall give directions by which they 

 may succeed in constructing a Leyden jar ; and by using 

 the electrophorus, previously described, t they may easily 

 try several of the experiments we are about to suggest. 



A Leyden jar may be made by half filling a thin, 

 round glass phial with brass or iron filings. A cork is 

 then to be fitted into its neck, and a wire should be fixed 

 n the cork, so that one end may enter the filings, and 

 r should extend two inches beyond the neck of 

 the bottle. On the outer end of the wire a brass knob 

 is to be fixed, or, what answers equally well, a round 

 cork covered with tinfoil The outside of the phial is to 

 be coated with tinfoil to half its height, and the bottom 

 should be similarly covered. The arrangement, though 

 somewhat rude ill it* construction, answers remarkably 



Sm-h a jar may easily be charged by moans of the 

 electrophorus, which must be first excited, as already 

 directed. t Tho cover is then to be brought to the knob 

 of the jar, when a spark will pass. This U to be re- 

 peated until the jar is charged, which may speedily bo 

 done with one of the kind we have just described. Tho 

 method of thus charging a jar with the electrophorus, 

 U illustrated in the following engraving. 



Having described the apparatus which will be required 

 f -r illustrating the laws of induction, <tc., we shall enter 

 into an experimental investigation of them, and also 

 SM , p. 179. t SM anu, p. 171. J See ant,, p. 1T2. 



illustrate the luminous, calorific, mechanical, and other 

 effects of a high charge of J .*. 54. 



electricity. 



When a Leyden jar is 

 charged by presenting the 

 knob to the coiidm : 

 an electrical machine in 

 :>, tin- natural equili- 

 brium of the latent el. 

 force is at once di*turl>ed, 

 and the surfaces acquire 

 what may be termed a polar 

 state. By this wo mean, 

 that a new and active con- 

 dition of force is imln 

 by which the hitherto quiescent and inactive conditions 

 of the latent electricities are placed in opposite i" 

 to i-:i'-h other, and yet having a tendency to recouiliine. 



This maybe better understood if we employ a diagram 

 illustrating the changes which lake place on the glass 

 surfaces; and instead of using the ordinary Ley.; 

 we shall suppose that we have, coated on each side with 

 tinfoil, a piece of window-glass. A Lcydeii jar, in f:u t, 

 may be regarded as such a piece of glass Kent into a 

 cylindrical form. We shall also confine our attention to 

 one small point on its surface, as such, of course, illus- 

 trates all that takes place in the entire jar whilst being 

 charged and discharged. 



In Fig. 25 we have two diagrams, of which No. 1 U 



intended to represent a jar or piece of coated glass, in 

 its natural state; and No. 2 illustrates the eject 

 change which takes place when the jar is charged, o is 

 the glass plate, and 6 c are the tinfoil coatings. 1 n N- 1 . 

 wo observe a circle illustrating an electric equilibrium. 

 in which the negative and positive, or vitreous and 

 resin. ms electricities, are in a state of combination. In 

 No. 2 we observe the effect of charging the jar, which 

 results in the separation of the two electricities, by which 

 a polar state is induced, anil each side of the glass plate 

 becomes in an opposite electrical state. 



If a metallic or other good conducting communication 

 be made between 6 aud c, in No. 2, the two electricities 

 will rccombine, the polar state be lost, and the coated 

 surfaces return to a state of equilibrium, such as at first 

 was spoken of, and shown in No. 1. 



We therefore observe, that the primary effect of 

 charging a glass surface, is to put the natural > 

 state of repose into an active condition ; and this results 

 in the separation of the two electricities, and their indi- 

 vidual appearance on each side of the glass plate, which 

 wo have supposed to be infinitely thin, and to have un- 

 dergone no electrical change of any kind in its mass or 

 substance. 



Wo need scarcely observe, that the explanation we 

 have given is that arising from the employment of the 

 double electric theory. If, however, we suppose the 

 existence of only one kind of electricity, then w 

 shall explain the changes which occur by stating, that 

 each side of the charged plate becomes polar; but that 

 the positive side has an excess of electricity, whilst the 

 negative one becomes deficient in that respect. These two 



