(118.) The following experiment of 

 Professor Richman, (the philosopher 

 who fell a sacrifice to his zeal for elec- 

 trical science by a stroke of lightning 

 from his apparatus,) is very instructive. 

 Let a pane of glass placed vertically, 

 and seen edgewise in fig. 29, be coated 

 on both sides, and furnished with two 



Fig. 29. 



igs alterna 



small electroscopes, p, n, consisting of 

 two pith-balls, one attached to each of 

 the coatings. Let the coating P be 



charged positively, while the coating N 

 is made to communicate with the ground. 

 The electroscope p will stand out from 

 the plate, and n will hang down close 

 to its coating, as long as N communi- 

 cates with the ground. But in propor- 

 tion as P loses electricity by gradual 

 dissipation in the air, the ball p will 

 gradually, but very slowly descend. If 

 we now insulate N, p will fall down at 

 first very speedily, and then more 

 slowly, till it reaches q, about half its 

 first elevation. The ball n will at the same 

 time rise to nearly the same height ; 

 the angle between the two electroscopes 

 continuing nearly the same as at first. 

 AVhen n has ceased to rise, both balls 

 will very slowly descend, till the charge 

 is lost by dissipation. If we touch N 

 during this descent, n will immediately 

 fall down, and p will as suddenly rise 

 nearly as much ; the angle between 

 the electroscopes continuing nearly the 

 same. Remove the finger from N, and 

 p will fall, and n rise, to nearly their 

 former places ; and the slow descent of 

 both will again recommence. The same 

 thing will happen if we touch P, p will 

 fall down close to the plate, and n will 

 rise to m, and so on ; and this alternate 

 touching of the coatings may be repeated 

 some hundreds of times before the 

 plate is entirely discharged. If we sus- 

 pend a crooked wire, bent, as shewn atW, 

 (Jig. 29,) having two pith-balls, from an 

 insulated point, s, above the plates, it 

 will vibrate with great rapidity, the 



ELECTRICITY. 



balls striking the coatin 

 and thus restoring the equilibrium by 

 steps ; each contact being attended by 

 a spark. 



(119.) If, instead of this gradual 

 discharge, a direct communication is 

 made between the two coatings by a 

 metallic wire extending from the one to 

 the other, the whole of the electric fluid 

 which was accumulated in the. positive 

 coating rushes with a sudden and violent 

 impetus along the conductor, and passes 

 into the negative coating, thus at once 

 restoring an almost complete equili- 

 brium, and rendering every part very 

 nearly, though not absolutely, neutral ; 

 for as there must always be some slight 

 difference in the quantity of electrical 

 charge in the two coatings, where one 

 of them is in communication with the 

 ground, there must always be a certain 

 excess, however minute, of electricity, 

 after the balance has been struck. 



(120.) This sudden transfer of a large 

 quantity of accumulated electricity is a 

 real explosion ; it gives rise to a vivid 

 flash of light, corresponding in intensity 

 to the magnitude of the charge. The 

 effect of its transmission is much greater 

 tli an that of the simple charge of the 

 prime conductor of the machine; for 

 while the latter gives a spark only, the 

 former imparts what is called an electric 

 shock, and the sensation it produces 

 when passing through any part of the 

 body is of a peculiar kind. We shall 

 describe their effects in a future chap- 

 ter ; at present we must confine our 

 attention to the purely electrical condi- 

 tions of the phenomenon. 



(121.) The presence of the coating is 

 not absolutely essential to the charge 

 and discharge for the tw r o surfaces of the 

 glass plate ; for if the glass be fur- 

 nished with moveable coatings, and 

 charged in the usual manner, upon re- 

 moving the coatings (taking care that 

 they be touched only by electrics,) the 

 greater part of the electricity will be 

 found to have attached itself to the sur- 

 faces of the glass plate, where they are 

 retained by their mutual inductive in- 

 fluence. In this state the charged plate 

 of glass may be gradually discharged 

 by making a communication between 

 its several parts in succession. It can- 

 not be discharged at once, for want of a 

 common intermedium for the simulta- 

 neous transference of the electricity of 

 the different parts of the surface. But 

 if this be supplied by replacing the 

 former coatings, or adding new ones, 



