EXCITATION OF ELECTRICITY. 
had it in. our power to exhibit the pheno- 
menon of the electricspark, which is electri- 
city producing ignition by breaking through 
a non-conductor, if we had not fortuitously 
experimented in circumstances where the 
electricity is first made to take the form of 
a charge, and afterwards brought into a 
state of considerable intensity, by sepa- 
rating those bodies from each other, which 
produced the compensation by their oppo- 
site states. Thus in the electrical machine, 
(see Nicholson, in the Philos. Trans. 1789,) 
little or no electric signs are produced by a 
cylinder rubbed by a very flat amalgamed 
leather, terminating in a neat line of con- 
tact. But this rubber and cylinder will, 
without any alteration, afford electricity, if 
a flat piece of metal, or the hand, or any 
other flat conductor, be held over that part 
of the cylinder which is in the act of re- 
ceding from the cushion, even though this 
conductor be held at the distance of an inch 
or more, without touching either the cy- 
linder or its rubber. It is proved from ex- 
periment, that the conducting body thus 
presented acquires the opposite state, and 
enables the cylinder to carry off a greater 
quantity of electricity in the form of a 
charge, the interposed air being the elec- 
tric. 
When the cushion is thick and rounded, 
as is the case with the human hand, which 
was first used for this purpose, the rounded 
part opposite the receding surface of the 
cylinder, performs the office of compensa- 
tion ; and the best application which has 
yet been made for this purpose, is that of a 
flap of silk proceeding from beneath the 
cushion, which assumes the negative state, 
so as to compensate the positive state on the 
cylinder, in a very considerable charge, 
which is conveyed by the rotation to the 
farther end of the silk, where it becomes 
uncompensated electricity upon the naked 
surface, at an intensity which could not 
otherwise have been produced. 
It has not yet been determined what are 
the conditions and circumstances of the 
change which takes place by the action of 
the air at the face of the rubber, nor why 
the surface of the glass should become posi- 
tive when rubbed with one kind of rubber, 
as for example, the human hand ; and nega- 
tive, if rubbed with another kind, such as 
cat-skin, or flannel, nor why glass, deprived 
of its polish, becomes negative with rubbers 
which would have rendered smooth glass 
positive. The most rational conjecture 
seems to be, that the surface which is most 
heated in consequence of its roughness, or 
the relative smallness of its dimensions, ac- 
quires the negative state. 
There is a certain velocity of rotation, 
which is about five feet per second, at 
which the excitation of electricity by a 
cylinder nearly vanishes ; but it returns 
again, the moment the velocity is diminish- 
ed. Some who maintain the existence of a 
material cause of heat, or caloric, are dis- 
posed to consider electricity as one of the 
states of caloric, in which the matter of heat 
can pass through bodies without raising 
their temperature, and with much greater 
velocity than that by which temperature is 
communicated. 
From the imperfect knowledge we pos- 
sess respecting excitation, it is very difficult 
tor the most experienced electricians to ex- 
cite a cylinder with certainty and power. 
If the cylinder be greased all over with 
tallow, and then turned for some time in 
contact with the cushion, the silk flap be- 
ing thrown back, and an amalgamed leather 
be applied and rubbed about upon the sur- 
face of the cylinder in motion, electric 
sparks are soon produced in abundance ; 
and if the silk be then thrown again into 
contact with the cylinder the excitation 
will, in general, be strong; but it is seldom 
so strong at the first time of exciting as it 
proves to be after the expiration of a day 
or more. It seems as if the amalgam and 
tallow required a considerable time of 
working to be brought into the best state 
for excitation. 
In order to judge of the degree of inten- 
sity of an excited cylinder, we must have 
recourse to some standard of the quantity 
of effect produced by the friction of a given 
surface. It has not been shewn that much, 
if any thing, depends on the thickness of 
the glass, though some kinds of glass are 
more excitable than others, and some not 
at all so. If a coated electric jar be taken 
of about one-twentieth of an inch in thick- 
ness (see Jar, electric), a cylinder or plate, 
moderately excited, will require fifty or 
sixty square feet to pass the cushion, in 
order to charge one foot of the coated glass, 
so as to explode over a rim of three inches, 
which is as much as can be admitted with- 
out danger of the explosion breaking 
through the jar. If the excitation be 
stronger, the charge may be made by the 
friction of thirty feet to one of the jar ; and 
the strongest excitation the editor has ever 
known, has been by the friction of fourteen 
square feet of a cylinder to charge one foot 
