441 
ELECTRIC I T Y. 
e'eflrk fluid imparted by tbe metallic plate, and by ap¬ 
parently condenfing and increafing its intenfity, he ulti¬ 
mately fncceeded in producing marked figns of difturb¬ 
ance, The conftrudtion of this apparatus is ihewn in the 
fame figure ; where a circular brafs plate, b, about four 
inches in diameter, is foldered to the brafs cap' of Ben- 
net’s eledtrofcope, O ; and, at a right angle with the 
plane of the cat), a fimilar brafs plate, a, connected with a 
brafs rod, F, and moveable about the joint, g, completes 
the apparatus. The latter plate, a, is non-infulated, and 
is never in contadt with the plate b, being placed at fuch 
a diftance as to allow a thick card to pais between. Con- 
fequently there w ill be a plate of air between a and b; 
and the charging of this plate of air is precifely on the 
principles defcribed in the explanation of the Leyden 
phial. 
Fig. 25, in the fame engraving, reprefents two metal¬ 
lic plates, A and B, about fix inches in diameter, with 
infulating 'handles, as at CC. One of thefe plates is made 
of zinc, and the other of copper. When they are brought 
frequently into contadt, and when, on each feparation, 
one of them, for infiance, the zinc plate, is, by its infu¬ 
lating handle, made to touch the cap of the eledtrofcope 
O, fig. 24, each portion of the eledtricity communicated 
to the cap, becomes equally ditfufed throughout the 
whole metallic medium in contact with it, the plate, b, 
participating of the efl'edt. The furrounding air is to 
feebly adted on by this very fmall intenfity, that unlefs- 
jts refiftance be confiderably diminifhed, no fenfible ef¬ 
fects will be produced. The very fmall plate of air be¬ 
tween the two plates ab, becoming influenced by this 
weak ftate of eledtricity, the proportion of the fluid it 
receives on the fide oppofed to the plate b, it gives out 
at the fide contiguous to the plate a, which, not being 
infulated, admits it with facility. In this way a may be 
conlidered as the coating of the outfide of the jar, and b 
as the infide coating. On every fuccefiive charge tire 
plate of air becomes additionally acted upon. So long 
as the plates a and b are preferved in this date, no fepa¬ 
ration of the gold leaves, reprefented oppofite c, takes 
place. When a Leyden phial is charged, and then placed 
on an infulating ftand, if a communication be formed 
with the external coating and an eledtrofcope, no difturb- 
ance will take place. The air in the external furface be¬ 
ing in a precife balance with the air on the internal fur- 
face, no change can take place on one fide fo long as the 
other fide is kept infulated. Thus it is with the con- 
denfer : the inftant the plate a is feparated from the plate 
b, the eledtricity which had been imparted and received 
by the intervening plate of air, being now expofed to the 
atmofpheric mafs, moves in the direction in which it 
meets with the leaf! refiftance. This direction is the me¬ 
tallic cap of the eledtrofcope ; and, according to the 
quantity thus regurgitating, the gold leaves c, become di¬ 
verged. The ftate of eledtricity is always pofitive, if 
the zinc plate be applied ; and negative, if the applica¬ 
tion of the copper-plate be made. 
If either the zinc or the copper-plate, be applied to 
the part of the cap n, which is diredtly oppofite to the 
plate b, the effects will be (Longer than when it is applied 
nearer to the plate b. This obfervation was firlt made 
by Mr. Read; and feems to accord with Volta’s notion 
as to moto-eledfric power. It is a further confirmation 
of the idea, that the real condudfors of eledtricity are re¬ 
plete with that principle ; and that the fir ft effedts pro¬ 
duced, are occasioned by the difturbance of the natural 
eledtricity of the body. It is well known to be a com¬ 
mon property of all univerfally diffufed- fluids, to be 
more powerfully adted upon in the part which is in a line 
of diredtion with the impelling power. This is the cafe 
with the tranfmiflion of found through folids or fluids, 
and alfo with the apparent motion of light. The fuc- 
ceflive impulfes adding at n, the natural electricity of the 
plate b, will alfo more powerfully difturb the intervening 
plate of air, than when the contadt is made at any other 
Vou VI. No. 363, 
part of the cap. In fuch a cafe, the motion communi¬ 
cated to the eledtricity of the plate b, will be weakened 
in proportion to the obliquity of the diredtion. This 
fa 61 is eafiiy proved, by the greater number of contadfs 
requilite in one diredtion than in the other. It would 
perhaps be more corredt to call this inftrument a galva- 
vifcopc, feeing that it does not fo corredtly meafure the 
powers of a battery as one hereafter mentioned, which 
may therefore he more properly regarded as a galvano¬ 
meter. When two diflimilar metals are employed, fuch 
as a plate of lilver and a plate of zinc, after frequent ufe, 
the zinc becomes changed on its furface, lofes its metal¬ 
lic fplendour, and has an oxydated appearance. The ex¬ 
periments of A (he and Fabroni alfo tend to prove certain 
chemical changes effected by the contadt of two dillimi- 
lar metals with an interpofed fluid. Suppofe Z, fig. 26, 
in the Eledtricity Plate III. to reprefent a plate of zinc ; 
S, a plate of filver; and F, a prepared frog lying between 
them. Upon completing the circuit between the filver 
and zinc plates, by means of the condudtor C, the muf- 
cles of the frog are inftantly thrown into violent convul- 
fions; and thefe adtions are repeated as often as the con¬ 
tadt is made, or the circuit complete, by the condudting 
wire and the two metals. When the prepared frog is in 
a deficcated ftate, without any humidity on the portions 
in contadt with the two plates, no contradtions enfue. 
The mufcular fibres of a frog are (o extremely fenfible 
to the influence of electricity, as to form the mod deli¬ 
cate eledtrofcope hitherto known. The great fallibility 
of the gold-leaf eledtrofcope of Mr. Bennet, is well 
know'll ; but, notwithftanding the improvement of Mr. 
Read’s condenfer, it evinces no difturbance by the Am¬ 
ple contadt of two diflimilar metals. But when the plate 
ot zinc, and the fimilar plate of copper, with infulating 
handles, as fhewn at fig. 25, are brought into contact 
about twenty times, and one of the plates applied, on 
each contadt, to the condenfer; upon removing the lat¬ 
ter, the leaves of gold are feen to diverge. The (late of 
eledtricity with which they diverge, is invariably pofi¬ 
tive, if the zinc plate be applied to the eledtrofcope ; 
and negative, if the application be on the fide of the cop¬ 
per-plate. I11 this cafe, the plates muft be at leaft five or 
(lx inches in diameter. Yet we fee, on the other hand,, 
that a very minute portion of zinc and filver will pro¬ 
duce violent convuifions in the limbs of a frog. 
If ten perfons moitten their hands with fait and water, 
and form a circuit, by holding the hands of each other;., 
and if, in this circuit, a prepared frog be placed, one of 
the perfons holding the animal by the feet, and another 
by the (pine, at the fame time that the firfi perfon in the. 
circuit has in his difengaged hand a half-crown, and the 
laft perfon a plate of zinc heid in the fame way, each 
time the two metals are brought in contadt, the frog will, 
become convulfed. 
Gren has remarked how the fenfations of feeing and 
tailing may at the fame time be produced, by a very 
(hiking experiment, which is as follows: “ The mufcu¬ 
lar motion, as well as the fenfations of feeing and tak¬ 
ing, may be produced at the fame inftant, by means of 
the electric (beam. Let four perfons ftand upon a fome- 
what imperfedt conductor, and communicate between 
themfelves in this way: let the firfi perfon hold in his. 
right hand, well moiftened, a cylinder of zinc, .and touch 
with the finger of his left hand the point of the finger 
of the fecond perfon, who, again, fliould touch with his 
finger the bare eye ball of the third : now, let the latter 
hold the legs of a prepared frog in his well-mdiftened, 
hand, at the fame inftant that the fourth perfon, w ith his 
moiftened right hand, lays hold of the rump of the frog, 
holding in his moiftened left hand a cylindrical piece of 
filver. If the firft and laft of the perfons, who form this, 
feries bring this zinc and filver into co.madt, and in this, 
way complete the circuit, the perfon whole tongue is 
touched will perceive an acidulous tafte ; the touched 
eye of the third perfon will perceive a fiafti of light; and 
5 U ihe 
