ON STANDARDS OF ELECTRICAL RESISTANCE. 121 
were to have been kept as permanent records of the resistance’of the copper coil 
on each occasion; but this resistance was found to vary so rapidly that the 
little copies could not be accurately adjusted with sufficient rapidity, and the 
resistance of the copper coil was therefore simply measured at the beginning 
and end of each experiment, in terms of an arbitrary unit. This propor- 
tional measurement was made with rapidity and precision by a new method, 
which, it is believed, is superior to the usual plan depending on the division 
or calibration of a comparatively short wire in the Wheatstone balance. (Ap- 
pendix D. Part II.) 
One unforeseen difficulty was caused by the change of direction of the 
earth’s magnetic force during each experiment. Our method is indeed inde- 
pendent of the intensity of the earth’s magnetism, but depends essentially on 
its direction, since it depends on the value of a deflection from the magnetic 
meridian. When this source of error was discovered by the continual and 
gradual change of zero observed, the absolute time of each experiment was 
noted, and a continuous correction obtained from the contemporaneous records 
at Kew, which agreed closely with the total changes observed at the beginning 
and end of each experiment. As the change of zero frequently reached three 
or four divisions in the course of the day, and as the whole deflection seldom 
exceeded 300 divisions, the importance of this correction is apparent. 
The presence of stationary masses of iron does not affect the experiments 
injuriously, so long as the uniformity of the magnetic field in which the coil 
resolves is undisturbed—a point carefully tested before the experiments 
began ; but a change in the position of iron in the neighbourhood during any 
experiment produces a corresponding error in the result, and the serious 
effect of moving very small masses of iron at a great distance from the coil 
was only fully appreciated in the later experiments. 
When it is considered that the method described is the simplest known, the 
discrepancy between the few determinations hitherto made in absolute mea- 
surement will cause no surprise. The time, labour, and money required 
could hardly be expected to be given by any one person, and in researches of 
this kind the value of the cooperation secured by the committees of the 
Association is especially evident. 
The absolute unit of the Sub-Committee is about eight per cent. larger than 
the unit as derived from a German-silver coil lately measured by Professor 
Weber. It is about six and a half per cent. larger than the unit as derived 
from a value published by Professor Weber of Dr. Siemens’s mercury units. 
It is about five per cent. smaller than the unit as derived from coils issued by 
Professor Thomson in 1858, based on Jacobi’s standard and a previous deter- 
mination by Professor Weber. It is about five per cent. smaller than Thom- 
son’s determination from Joule’s silver wire. It agrees most closely with an 
old determination of a copper standard made by Weber for Professor Thomson, 
which it exceeds by only a very small fraction. 
The experiments of the Sub-Committee agree much better than the above 
one with another. Owing to the gradual improvement in the method and 
apparatus, the experiments of the last three days are alone considered satis- 
factory. On the first day the maximum deviation in six distinct experiments 
from their mean result was 2:4 per cent. On the second day the maximum 
deviation in four experiments from their mean was 1:3 per cent. On the 
third day the maximum deviation in five experiments from their mean was 
1-15 per cent. The maximum deviation in the means of the three days’ ex- 
periments from the mean of the whole is only four-tenths per cent. 
These results are not unsatisfactory, and are perhaps more accurate than 
