28 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
Historical Introduction. 
There are many methods by which a resistance can be measured absolutely in the 
electromagnetic system of units, and all of these necessarily involve absolute measure¬ 
ments of a length and of a time. The length may be the axial length or radius of a 
coil, or the radius of a disc, or it may involve all of these, and the time may be the 
time of vibration of a magnet, or of a rotation of a coil, or of the period of an 
alternating current. In any case, the precision secured in the measurement of a 
resistance depends primarily on the accuracy obtained in these measurements of 
length and of time. 
The first absolute measurements* of a resistance were made by KiRCHHOFrt in 
1849, but it is to W. Weber| that we owe the first distinct proposal (in 1851) of a 
definite system of electrical measurements according to which resistance can be 
measured in terms of an absolute velocity. 
Weber devised three methods by which the resistance of a wire can be determined 
absolutely, and he published the results of experiments by two of these. The first 
was by means of an earth inductor, and the second by observing the damping of a 
swinging magnet, the results obtained differing among themselves by 5 parts in 1900. 
In 1853§ he made a determination of the specific resistance of copper, but the experi¬ 
ments were made more to develop the methods than for exact measurements. 
In 1862 Weber 11 made a more exact determination of resistance using a method 
compounded of his first two methods and eliminating the constant of the galvano¬ 
meter. The results of these experiments were embodied in a determination of the 
value of the Siemens unit and of a standard coil sent to him by Sir William 
Thomson, but the unit obtained was about 8 per cent, less than the 1863 unit of the 
British Association Committee on Electrical Standards.^ 
The measurements made by Maxwell, Fleeming Jenkin, and Balfour Stewart 
for the British Association Committee, although subsequently found to be incorrect 
by nearly 1'5 per cent., are the first with any claim to precision. The method 
adopted by these experimenters is that of the rotating coil and was devised by 
Prof. Thomson (later Lord Kelvin) independently, we believe, of a prior suggestion 
by W. Weber. The apparatus consists of a short-circuited coil rotating about a 
* Admirable summaries and criticisms of various methods will be found in Wiedemann’s ‘ Electricitat,’ 
vol. IV., p. 910; Mascart and Joubert’s ‘Lemons sur l’Electricite,’ II., p. 581; Lord Rayleigh, ‘Phil. 
Mag.,’1882; Wiedemann, ‘Phil. Mag.,’1882; Glazebrook, ‘Electrician,’ 1890; Dorn, ‘ Wissenschaft- 
liche Abhand. der Phys. Techn. Reichsanstalt,’ vol. II., p. 357, 1895. 
f “Bestimmung der Constanten von welcher die Intensitat inducirter elektrischer Strome abhangt,” 
‘Pogg. Ann.,’ Bd. 76, S. 412. 
| ‘Elektrodynamische Maassbestimmungen,’ or ‘Pogg. Ann.,’ Bd. 82, S. 337. 
§ ‘ Abh. d. Kon. Ges. d. Wissenschaften zu Gottingen,’ Bd. 5. 
|| ‘Zur Galvanometrie,’ Gottingen, 1862. 
11 ‘Reports of B.A. Electrical Standards Committee,’ 1863. 
