ON STANDARDS OF ELECTRICAL RESISTANCE. 123 
that the British Association units shall be represented by several equal stan- 
dards made of the different metals, which, so far as our limited experience 
goes, show the greatest signs of constancy. Two at least of those standards 
would be made of mercury, in the manner proposed by Dr. Siemens. The 
permanent agreement between several of these standards would afford the 
strongest possible proof of their constancy. 
Passing to other electrical measurements, the Committee have to report that 
Professor W. Thomson has successfully constructed a material standard gauge 
by which electromotive force or difference of potentials can be directly mea- 
sured. This instrument is founded on a measurement of the electrical attrac- 
tion exerted on a small moveable portion of a large conducting-plane by 
another large parallel plane fixed at a constant distance, and electrified to a 
different potential. The force exerted is ultimately measured by the torsion 
of a platinum wire ; but the difference of potential corresponding to any one 
gauge is simply indicated by the motion of an index to a sighted position. 
If the planes are brought sufficiently close, with a given torsion in the 
platinum wire, the moveable’ piece will be in a condition of unstable equili- 
brium when its index is in the sighted position, but if moved to a greater 
distance the equilibrium will be stable; hence, by a correct choice of the 
distance between the two planes, or initial torsion in the platinum wire, as 
compared with the difference of potential to be measured, any required 
delicacy of indication is obtained. The constancy of the gauge, like that 
of all standards, depends simply on the constancy of the materials of which 
it is constructed, and there is no reason to apprehend any special difficulty 
in the present case. 
Professor Thomson has also on the same principle constructed an electro- 
meter in which the distance between the parallel planes is made variable, 
and is adjusted by a micrometer-screw. The plane conductor, of which the 
small moveable index forms part, is in this instrument permanently main- 
tained at a high potential by connexion with the inner coating of a Leyden 
jar, and the other plane is connected with the body to be tested. Calcula- 
tion, confirmed by experiment, shows that in these instruments the difference 
of potentials between any two bodies, successively tested, is directly pro- 
portional to the difference of the distances between the parallel planes 
required in each case to bring the index to its sighted position. This 
difference of distance is the same whatever be the charge of the Leyden 
jar, provided only it remains constant during the comparison of the two 
bodies. With this limitation, the indications of the instrument may be 
called independent of the charge of the Leyden jar. There can be little 
doubt that gauges of electromotive force and electrometers, fulfilling the 
above conditions, will shortly become as necessary to all practical electricians 
as standards of resistance and sets of resistance-coils. 
No progress has been made in the measurement of currents, and much 
remains to be done in this respect. The method already described, depending on 
the use of a tangent galvanometer, requires a knowledge of the horizontal force 
of the earth’s magnetism, and is, therefore, in most cases beyond the reach of 
observers where greater accuracy is required than can be obtained by taking 
their value from the scientific almanacs. Next year it is hoped that this 
Want may be remedied, and the present Report may fitly conclude by the 
enumeration of objects to be pursued by the Committee, if reappointed at the 
present Meeting :— 
ist. The experiments on the determination of the absolute unit of resist- 
ance will be continued. 
