ON STANDARDS OF ELECTRICAL RESISTANCE. 163 
- Part V.—Eercrricat MsasvREMENTS DERIVED FROM THE Five ELEMENTARY 
MEASUREMENTS, 
47. Electric potential. 53. Electrochemical equivalents. 
48. Density—resultant electric foree— | 54, Electromotive force of chemical 
electric pressure. affinity. 
49, Tension. 55. Tables of dimensions and other con- 
50. Conducting power, specific resist- stants. 
ance, and specific conducting | 56, Note to the table of dimensions, by 
power. Professor Clerk Maxwell. 
51. Specific inductive capacity. 57. Magnitude of units and nomen- 
52 Heat produced in a conductor by a clature, 
current, 
Apprnvix D.— Description of an Experimental Measurement of Electrical 
Resistance, made at King’s College. By Professor J, Cherx Maxweut and 
Messrs, Banrour Srewarr and Fiermine Jenxxn. 
Parts I., IIT., and IV., by Professor Maxwett. 
Part II., by Mr. Freemine Jenxry. 
Part I. General description of the method employed.—In the general 
Report of the Committee, and in Appendix OC, it has already been shown that 
the most important aid to the exact science of electricity would be the de- 
termination of the resistance of a wire in absolute measure, and the publica- 
tion of standards of resistance derived from this wire. This has already 
been done by Weber*; butit is desirable that the determination of a quantity 
so important should not be left in the hands of a single person. 
Weber has employed two methods. 
Ist. By suddenly turning a coil of wire about an axis so as to alter its 
_ position relatively to the terrestrial magnetic lines of force, he produced an 
electromotive force acting for a short time in the coil. This coil was con- 
‘nected with another fixed coil having a magnet suspended in its centre. The 
current generated by the electromotive force passed through both coils and 
gave the magnet a sudden impulse, the amount of which was measured by 
its extreme deflection. 
Thus an electromotive force of short duration produced a current of short 
duration. The total amount of electromotive force depended on the size of 
the moveable coil and on the intensity of terrestrial magnetism. The total 
amount of the current is measured by the impulse given to the magnet, and 
the mechanical value of the impulse is measured by the angle through which 
it swings. The resistance of the whole circuit, consisting of both coils, is 
then ascertained by dividing the electromotive force by the current. 
2nd. Weber’s second method consisted in causing a powerful magnet to 
oscillate within a coil of wire. By the motion of the magnet currents are 
ee in the coil, and these, reacting on the magnet, retard its motion. 
e rate of diminution of the amplitude of the oscillations, when compared 
with the rate of diminution when the circuit is broken, affords the means of 
determining the resistance of the circuit. 
Professor W. Thomson has designed an apparatus by which the resistance of 
a coil can be determined in electromagnetic measure by the observation of 
the constant deflection of a magnet, and his arrangement has been adopted 
for the experiments made by the Committee. 
* Pogg. Ann. Bd. 82. p. 337 (March 1851); Electrische Maasbestimmungen, Leipzig, 
Wiedemann ; Memoirs of the Royal Society of Sciences of Saxony, vol. i. p. 197; and 
Phil. Mag. 1861. 5 
M 
