ON STANDARDS OF ELECTRICAL RESISTANCE. 147 
force cut by the circuit, and he has also described the state of a conductor in 
a field of force as a state the change of which is a cause of currents. He calls 
it the electrotonic state, and, as we have just seen, the electrotonic state may 
be measured by the number of lines of force which pass through the circuit 
at any time. 
The measure of electromotive force used by W. Weber, and derived by him 
(independently of the principle of conservation of energy) from the motion of 
a conductor in a magnetic field, is the same as that at which we have arrived ; 
for, from equation (15), we find that the unit electromotive force will be pro- 
duced by motion in a magnetic field when one line of force is added (or sub- 
tracted) per unit of time, and this will occur when in a field of unit intensity 
a straight bar of unit length, forming part of a circuit otherwise at rest, is 
moyed with unit velocity perpendicularly to the lines of force and to its own 
direction, 
To W. Weber, whose numerical determinations of electrical magnitudes are 
the starting-point of exact science in electricity, we owe this, the first defini- 
tion of the unit of electromotive force; but to Professor Helmholtz* and to 
Professor W. Thomsont, working independently of each other, we owe the 
proof of the necessary existence of magneto-electric induction and the deter- 
mination of electromotive force on strictly mechanical principles. 
32. On Material Standards for the Measurement of Electrical Magnitudes.— 
The comparison between two different electrical magnitudes of the same na- 
ture, e. g. between two currents or between two resistances, is in all cases 
much simpler than the direct measurements of these magnitudes in terms of 
time, mass, and space, as described in the foregoing pages. Much labour is, 
therefore, saved by the use of standards of each magnitude ; and the construc- 
tion and diffusion of those standards form part of the duties of the Committee, 
Electric currents are most simply compared by ‘‘electro-dynamometers” (20) 
—instruments which, unlike galvanometers, are practically independent of the 
intensity of the earth’s magnetism. When an instrument of this kind has 
been constructed, with which the values of the currents corresponding to 
each deflection haye been measured (19, 20), other instruments may easily 
be so compared with this standard, that the relative value of the deflections 
produced by equal currents on the standard and the copies shall be known, 
Hence the absolute value of the current indicated by each deflection of each 
copy will be known in absolute measure. In other words, in order to obtain 
the electromagnetic measure of a current in the system described, each obser- 
ver in possession of an electro-dynamometer which has been compared with 
the standard instrument will simply multiply by a constant number the de- 
flection produced by the current on his instrument (or the tangent or sine of 
the deflection, according to the particular construction of the instrument). 
Electric quantities may be compared by the swing of the needle of a gal- 
yanometer of any kind. They may be measured by any one in possession of a 
standard electro-dynamometer, or resistance-coil, since the observer will then 
bein a position directly to determine C, in equation (12), or R, in equation (14). 
Capacities may be compared by the methods described (26); and a Leyden 
jar or condenser (41) of unit capacity, and copies deriyed from it, may be pre- 
pared and distributed. The owner of such a condenser, if he can measure 
electromotive force, can determine the quantity in his condenser, 
* Paper read before the Physical Society of Berlin, 1847 (vide Taylor’s Scientific 
Memoirs, part ii. Feb. 1853, p. 114). 
+ Transactions of the British Association, 1848; Phil. Mag. Dec, 1851, ; 
L 
