(841.) 
Its merits 
and de- 
fects. 
Cuar. VIL., § 6.] 
Munich. He died on the 7th July 1854. His 
theory of electrical conduction was not highly appre- 
ciated in Germany until it had received, in 1841, an. 
eminent mark of approval from the Royal Society of 
London, by the award of their Copley medal. His 
principal work on the Galvanie Cireuit (Die galvan- 
ische Kette mathematisch bearbeitet; Berlin, 1827) 
has had a somewhat peculiar fate. Accepted by only 
a few persons as a great discovery, it met with com- 
paratively little attention, at least until recently ; yet 
notwithstanding the long anticipation by Ohm of his 
results, it has been his misfortune to have their ori- 
ginality contested, 
It seems not difficult to account for the diversity 
of estimation in which this work has been held. The 
primary fault is the author’s own, He deduces the 
strength of a voltaic current in any given circuit, and 
the electroscopic excitement of each part of the cir- 
cuit, by means of reasoning seemingly a priori, from 
certain assumed axioms submitted to mathematical 
reasoning. The axioms are very simple; the theory 
founded on them is intended to correspond to Fourier’s 
theory of heat, of which, indeed, in point of form, it is 
a mere and literal copy; but as every circumstance 
which introduces real complication is soon left on 
one side, the leading propositions are almost self- 
evident results of the axioms. In short, the pa- 
rade of mathematics is uncalled for, and the whole 
structure of the theory seems so slight and ques- 
tionable that one is surprised that it should ever 
have been regarded as more than a clever expres- 
sion of some approximately true experimental laws. 
It appears, indeed, that this is the simple fact ;— 
that the axioms were obtained from the results 
which they seem to predict, and that Ohm was an 
experimentalist before he became an author. In 
this guise we understand how to treat the so-called 
“Laws of Ohm.” ‘They are truly important empi- 
rical laws, calculated to guide the practical man in 
applying and measuring galvanic forces, to enable 
the theorist to form clearer notions of the different 
(often confusing) effects of these forces, and to reduce 
their varying energy to calculation; but we must be 
allowed to doubt whether Ohm has thrown any new 
light on the real first axioms of electrical excitement 
or transmission. 
The most important of these laws refer to the 
numerical measure of the voltaic stream circulating 
in the conductor of a closed circuit. Such a closed 
circuit may be imagined to consist of—(1.) an excitor 
or battery; (2.) a conductor homogeneous or other- 
wise, but necessarily continuous, uniting the ends of 
the battery. The eweiting force is derived (we will 
assume) from the chemical or thermo-clectric action 
present in the battery. The electric equilibrium 
being disturbed, is restored more or less speedily 
through the medium of the conductor which connects 
the poles. If the conductor be good, the electricity 
passes rapidly through it, and does not accumulate in 
ELECTRICITY.—-OHM—DANIELL, 
983 
the battery ; if the reverse, it accumulates until it ac- 
quires power to overcome the resistance, and then it 
passes through in a stream less abundant, but of a 
higher intensity. If the construction of the battery 
does not permit that degree of intensity to be reach- 
ed, the electricity stagnates in the battery, the con- 
ductor cannot perform its office, no effect results. The Ilustra- 
whole maybe compared to a spout of water discharged ti” 
into a trough, from the bottom of which extends a 
long narrow horizontal pipe. The water is the elec- 
tricity, the trough is the battery, the pipe is the con- 
ductor, If the pipe be very long and narrow, no 
water at. all will pass through it until the water in 
the trough has attained a certain height, or has a 
head of pressure sufficient to overcome the resistance 
in the pipe. If the trough be filled to the brim 
without the resistance being overcome, the trough is 
as good as plugged, no motion takes place, the stream 
regurgitates, The longer the pipe the feebler the 
stream that passes; shorten the pipe indefinitely, 
and the efflux depends only on the construction of 
the trough. Indeed, the illustration might be pushed 
considerably farther. The depth of the cistern re- 
presents the electro-motive force of the voltaic com- 
bination ; its area the size of the plates, By in- 
creasing the latter, we do not give the means of 
overcoming more resistance; but when the resist- 
ance is small, we afford a larger supply without 
lowering the level—i. ¢., the intensity. 
Ohm regards the current as proportional to the elec- 
(843.) 
tro-motive force directly, and to the resistances in- eee 
versely ; and the latter are divided into (a) the re- hina: 
sistance of the battery itself to the passage of the cur- 
rent; (b) the resistance of the conductor. Now the 
latter varies as the length of the wire completing the 
circuit. We may therefore double its amount by 
doubling the length of wire joining the poles; and 
if we observe the strength of the current passing 
before and after this has been done, we have a mea- 
sure of a+ in the first experiment, and of a+26 
in the second ; and 6 being assumed to be known, a, 
or the comparative resistance within the battery, 
becomes known also. 
The resistance of a standard copper wire a foot 844.) 
long may be taken as the unit of resistance, 
Wheatstone finds it convenient to assume a copper 
wire a foot of which weighs 100 grains. M, Jacobi 
prefers a métre of copper wire one millimétre in dia- 
meter. The resistance is as the length, and inversely 
as the sectional area, 
Mr Standard 
of resist- 
ance. 
To measure the current two methods have chiefly 945.) 
been used, and the results agree closely. One is Force of 
the tangent compass. 
to pass through a thick wire arranged in a vertical 
circle, At the centre of the circle is placed a very 
short magnetic needle. When a current passes the 
needle is deflected ; and it is easy to show that the 
deflecting forces are as the tangent of the angle of 
deflection. A double or treble force in the circuit 
A voltaic current is allowed the current 
—tangent 
compass. 
