ELECTRICITY. 



U 



Fig. 31. 



taring of the powder, and thus u soon M the jar is sufficiently 

 filled it dischargee itself, and at the tame time inflame* the 

 powder. In thin way we receive by the one experiment illus- 

 trations of electrical repubiion and of the heating, the luminous, 

 and the sonorous effects of the shook. 



In a former lesson we hare noticed a few more of the 

 mechanical effects ; oar readers, however, who have machines of 

 their own will soon find many more ex- 

 t han wo can refer to hero. 



Tint main points remaining for us to 

 f under tho head of Frictionul 

 icity arc the principles and con- 

 struction of condensers, and the varying 

 and remarkable phenomena of Atmo- 

 spheric Electricity. We proceed, at pre- 

 sent, to describe some of the former. 



When a charge of electricity is spread 

 over a largo surface, it becomes, of 

 course, diminished in intensity, and 

 therefore its effects are less clearly seen. 

 A condenser is on apparatus for col- 

 lecting this on a email surface, and 

 thin rendering its presence more per- 

 ceptible. Fig. 32 represents tho form 

 of this instrument in most common 

 use, and known as Epinus* 

 Condenser. In this the two 

 metallic plates, A and c, 

 are mounted on insulating 

 supports, so arranged as 

 to slide in a groove, and thus 

 vary their distances from 

 one another ; from each of 

 these a pith ball is sus- 

 pended to indicate tho pre- 

 sence of free electricity. A 

 glass plate, B, somewhat 

 larger than the discs, is also 

 supported on an insulating 

 stand between them. Tho 

 plato A is now connected 

 with the conductor of the machine by 

 means of a chain or piece of wire ; the 

 other plate, c, is connected with the 

 ground in a similar way. They are then 

 brought closer together, so that they 

 both are in contact with the glass 

 plate B. 



The plate A will first become charged 

 with positive electricity of tho same 

 tension as that of the conductor, and 

 would remain in that state were it not 

 for the presence of the plate c. The 

 fluid in A, however, acts by induction 

 on that in c, attracting the negative 

 portion to the side nearest the glass, 

 and causing tho positive to escape by 

 the wire to tho earth, c is thus nega- 

 tively charged, and therefore reacts by 

 induction on A, neutralising or render- 

 ing latent a portion of its charge. A 

 small amount of electricity is, however, 

 loft free upon it, owing to the thick- 

 ness of the glass interfering with the 

 action of induction. Still the tension 

 of this plate is much reduced and 



brought below that of tho machine ; a further supply of elec- 

 tricity therefore passes to it, and this produces a similar series 

 of effects, a portion of it being, as before, neutralised. In this 

 way a greatly increased amount of electricity is accumulated 

 in A. As long, however, as both plates remain against the glass, 

 tho only effect seen is a slight divergence of the pith ball 

 attached to it, which arises from tho portion of its charge which 

 is nnneutralised. 



Now remove the chains connecting the plates with the con- 

 ductor and the ground respectively. In 3oing this, the free 

 electricity will probably be removed, and both pendulums will 

 remain at rest. If, however, we separate the plates, so that the 



effect of induction is lost, the fluid on each will be set free, and 

 the divergence of the pith balls will at onoe thow that they are> 

 highly charged, the one with positive and the other with 

 negative electricity. They can now be discharged separately, 

 whereas, when in contact with the glass, this could only be done 

 by connecting them together, M in discharging a Leyden pan*. 

 The instrument may, in fact, be considered merely M a Leyden 

 pane with movable coatings, and affords 

 a farther explanation of toe action of 

 the pane or jar. 



The condensing electroscope (Fig. 83) 

 acts on the same principle as the eon- 

 denser, and is of great use in detect- 

 ing the presence of electricity, when in 

 too small quantities to affect the ordi- 

 nary gold-leaf electroscope. It consist* 

 essentially of a condensing plate, A, 

 which is fitted so as to be in metallic 

 communication with the gold leaves. 

 This plate is coated with a thin layer 

 of insulating varnish, which acts as 

 a di-electric, and takes the place of the 

 pane of glass in Epinus' Condenser. A 

 second disc, fitted with an insulating 

 handle, rests upon A; this is called 

 the collecting plate. 



Sometimes it is considered 

 more convenient to make the 

 condensing plate vertical in- 

 stead of horizontal ; and the 

 collecting plate is then fitted 

 on a glass rod hinged to the 

 stand, so that it can be 

 turned back when necessary. 

 Tho body tc be examined 

 is now touched by the col- 

 lecting plate, which is then 

 placed upon A, and the finger 

 placed against its under side ; 

 or the charged body may be 

 placed upon the collecting 

 plate, the finger being, as before, made 

 to touch the lower one, as shown in 

 Fig. 34. If the body was in any degree 

 charged, it will at once share its elec- 

 tricity with the plate. This will then 

 act by induction on A, and drive off a 

 corresponding amount of the fluid, 

 thereby neutralising the first charge ; 

 and in this way, by touching again, 

 nearly all the electricity from the sub- 

 stance will be collected on the disc and 

 held there by induction, the under plate 

 being charged to the same degree with 

 the opposite fluid. Now remove the 

 finger, and lift the disc by means of its 

 insulating handle. Induction will at 

 once cease, and the fluids will therefore 

 be set free and cause the leaves to di- 

 verge. It can easily be ascertained 

 whether they diverge with positive or 

 negative fluid by bringing a positively 

 charged body near the instrument, 

 and observing its effect. If the leaves 

 diverge more, the charge is positive; 

 if less, negative. The electricity of 

 the leaves is, it must be remembered, the reverse of that of the 

 body tested. 



With such sensitive apparatus as this, great care is required 

 to guard against drawing wrong conclusions. If the plates are 

 made of a metal which is easily oxidised, tho action of the air 

 will often bo sufficient to manifest faint signs of electricity. 

 Too much reliance must not, therefore, be placed on a single 

 experiment. For all ordinary purposes, however, an ordinajy 

 gold-leaf electroscope will be sufficiently delicate. 



We shall conclude our lessons on Fractional Electricity with 

 a single paper on Atmospheric Electricity, and then pass on to 

 the consideration of Voltaic Electricity. 



