(742.) 
Volta’s 
Straw 
Electrome- 
ter. 
(743.) 
Atmosphe- 
ric electri- 
city. 
962 
excited cake of pitch and resin. This charge is re- 
ceived by induction merely ; and as it expends none 
of the electricity connected with the pitch, the suc- 
cessive charges are precisely equal, a circumstance 
which enabled Volta very simply to give a nume- 
rical value to the amounts of electricity used in his 
experiments, as they were given by one, two, or 
more contacts with the electrophorus. The Con- 
denser, an ingenious and useful instrument for ac- 
cumulating small charges of electricity until they 
attain a measurable amount, or cause divergence in 
the electrometer, was described by Volta in 1782 in 
the Transactions of the Royal Society, its construction 
having occurred to him in following out the idea of 
the electrophorus. This instrument was ultimately 
of essential service in establishing his theory of Elec- 
tro-motion. The theory of both the electrophorus 
and condenser had been indicated by Apinus some 
time before Volta constructed them; but he did not 
apply them in the practical way which Volta did to 
the improvement of his science. Volta being, be- 
sides, unquestionably ignorant of Zpinus’s labours, 
has been generally and justly regarded as the real 
inventor. Though both of these instruments depend 
on the principle of induced electricity, Volta never 
appears to have possessed correct views on that sub- 
ject, but throughout his writings speaks of electrical 
atmospheres, and uses other phrases of the old school 
of electricians, showing a certain vagueness in his 
conception which a study of the writings of his able 
contemporaries, Aipinus and Coulomb, would have 
dissipated. : 
But Volta’s tact was unconnected with any tinge 
of mathematical reasoning: his experimental ability, 
his caution, and his persevering devotion to one sub- 
ject, enabled him, however, to advance science in a 
different way, Even the simple Straw Electrometer 
which he generally used, was tested by him with 
such skill and care as to lead to correct results in 
the measurement of small quantities of electricity. 
M. Biot has indeed criticised his preference for so rude 
an instrument, which depends, mathematically con- 
sidered, upon repulsions of a very complicated cha- 
racter; but as Volta carefully tested its comparabi- 
lity up to 30° of divergence, and found it proportional 
to the force, there is no doubt that he was justi- 
fied in relying on its use; and Arago, in opposition 
to his colleague, maintains that Volta’s essay on the 
Straw Electrometer is one of the best examples of 
experimental research which can be put into the stu- 
dent’s hands. This instrument is described in the 
first of a series of letters to Lichtenberg collected in 
the first volume of his works; the subject of these 
letters being the Electricity of the Atmosphere. 
The important experiments of Dalibard and Frank- 
lin, repeated with fatal consequences by Richmann, 
had demonstrated the perfect resemblance or rather 
MATHEMATICAL AND PHYSICAL SCIENCE. 
[Diss. VI. 
identity of lightning and electricity, Lemonnier dis- 
covered the fact of electricity being manifested when 
no thunderstorm threatened, and even when the sky 
was cloudless, and that it was subject to diurnal va- 
riations of intensity. Beccaria farther ascertained 
that with a clear atmosphere the electricity of the 
air is always positive. De Saussure, Deluc, and 
Volta continued the interesting enquiry. The 
first, availing himself of the known action of points 
to draw off electricity, connected his electrometer 
with a pointed rod two or three feet in length. 
Volta substituted for this the flame of a lamp pro- 
ducing a heated current of air, which has a won- 
derful power of drawing off electricity; and he sug- 
gested the employment of large fires during thunder- 
storms in preference to metallic conductors. Volta 
hesitates not to ascribe to the worshippers of Jupiter 
Tonans the secret intention to draw off the electricity 
of heaven by the action of the flames on the altar. 
Arago has ingeniously suggested that a statistical 
enquiry as to the frequency of thunderstorms in the 
neighbourhood of extensive iron-smelting furnaces 
might test the value of this safeguard. The straw 
electrometer which Volta connected with his appara- 
tus was capable (by a gradation of instruments of 
greater or less delicacy) of measuring numerically 
the force of charges from 1000 to 2000 units. 
That the chief source of atmospheric electricity is 
evaporation appears first to have occurred to Volta, 
and to have been first demonstrated by experiments 
made either with him or by his suggestion, at Paris 
in 1780, by Lavoisier and Laplace. The history is 
given by Volta himself in a paper in the Philosophical 
Transactions for 1782. When water is thrown upon 
an insulated heated body so that evaporation takes 
place, or when hot coals are thrown into an insulated 
vessel of water, the hot body is usually found to be 
electrified negatively. Volta has very candidly stated 
the inversions of effect which occasionally occur, and 
which still throw some doubt upon the precise signi- 
ficance of this very important experiment, Later 
experimenters have thought that absolutely pure 
water developes no electricity: this, however, will 
not affect the validity of the explanation of the origin 
of atmospheric electricity. To prevent misappre- 
hension it may be observed that the astonishing de- 
velopement of electricity from high-pressure steam 
escaping through a small aperture, as lately observed 
by Mr Armstrong, appears, from the experiments of 
Dr Faraday, to depend on an entirely different cause. 
Of Volta’s electrical theory of Hail we cannot now 
stop to speak, 
Volta, who understood chemistry and who always 
took a peculiar interest in the inflammable gases, ae 
contrived the Eudiometer which is often erroneously peter 
called Cavendish’s, having been frequently used by 
that philosopher. The amount of oxygen in the air 
1 Opere, vol. i., part 2., p. 205. Volta’s contrivance dates from the beginning of 1787. Bennett imagined it independently. 
(744) 
ue to eva- 
ration. 
(745.) 
olta’s 
dio- 
