152 REPORT—1863. 
by Coulomb. Simple forms of this kind may be termed absolute condensers. 
A sphere in an open space is such a condenser, and the quantity it contains 
is se (eq. 23). A more convenient form isa sphere of radius a, suspended 
in the centre of a hollow sphere, radius y, the latter being in communication 
with the earth. The capacity, s, of the internal sphere is then, by calculation, 
=a 
‘Tyee > oii, ltt US ide my 
By a series of condensers of increasing capacity, we may measure the capacity 
of any condenser, however large. The comparison is made by the method 
described above (40). Thus, the practical method of measuring quantity in 
electrostatic measure is first to determine the capacity of the conductor con- 
taining the charge, and then to multiply that capacity by the electromotive 
force producing the charge (43). 
42. Practical Measurement of Currents.—The electrostatic value of currents 
can be obtained from equation (21), when ¢ and rv are known, or from equa- 
tion (19), when v and C are known, or by comparison with a succession of 
discharges of known quantities from an absolute condenser. 
43. Practical Measurement of Electromotive Force.—The relations expressed 
by eq. (17) and (23) show that in any given circumstances the force exerted 
between two bodies due to the effects of statical electricity will be proportional 
to the electromotive force or difference of potential (47) between them. This 
fact allows us to construct gauges of electromotive force, or instruments so 
arranged that a given electromotive force between two parts of the apparatus 
brings an index into a sighted position. In order that the gauge should serve 
to measure the electromotive force absolutely, it is necessary that two things 
should be known: first, the distribution of the electricity over the two attract- 
ing or repelling masses (or, in other words, the capacity of each part); secondly, 
the absolute force exerted between them. For simple forms, the distribution, 
or capacity of each part can be calculated from the fundamental principles 
(33); the force actually exerted can be weighed by a balance. By these 
means Professor W. Thomson* determined the electromotive force of a 
Daniell’s cell to be 0:0021 in British electrostatic units, or 0:0002951 in 
metrical units. This proposition is equivalent to saying that two balls of a 
metre radius, at a distance d apart in a large open space, and in connexion 
with the opposite poles of a Danicll’s cell, would attract one another with a 
0-0002951 0:0000239 
i re 
An apparatus by which such a measurement as the foregoing can be carried 
out is called an absolute electrometer. It will be observed that, although 
the definition of electromotive force is founded on the idea of work, its 
practical measurement is effected by observing a force, inasmuch as when 
this force exerted between two conductors of simple shape is known, the work 
which the passage of a unit of electricity between them would perform may 
be calculated by known laws. 
44. Comparison of Electromotive Forces by their Statical Effects.—This 
comparison is simpler than the absolute measurement, inasmuch as it is not 
necessary, in comparing two forces, to know the absolute values of either. In- 
struments by which the comparison can be made are called electrometers. 
Their arrangement is of necessity such that the force exerted between two 
given parts of the instrument shall be proportional to the difference of potential 
* Paper read before the Royal Society, February 1860. Vide Proceedings of the Royal 
Society, vol. x. p- 318, and Phil. Mag, vol. xx, 4th Ser. 1860, p- 233. 
force equal to absolute units, or gramme weight. 
