358 
LORD RAYLEIGH OH THE CONSTANT OF MAGNETIC 
20. The results of all the observations (other than preliminary) which were thought 
worthy of reduction are exhibited in the accompanying tables, grouped in three series. 
In Series I., II. the first tube was employed; the principal difference between them 
being that in Series I. the light traversed the tube three times , while in Series II. the 
light passed but once. It will be seen that in Series I. the actual double rotation 
varied from about 9° to 19°, and the currents from about ■§ ampere to 1 ampere. In 
Series II. stronger currents were usually passed, amounting to about 1J ampere, but 
the rotation was only about 9°. The extreme deviation from the mean is only about 
*4 per cent., if we exclude the observations of May 29, which owing to interruptions 
and other causes were marked as unsatisfactory before reduction. 
The Nicol was used as analyser in Series I., and on June 3 of Series II. The 
remaining observations of Series II. and the whole of Series III. were taken with a 
double-image prism, read in all four positions as already explained by the example of 
July 25. 
For the observations of Series III. the second tube was employed, with some improve¬ 
ments in the provision against the communication of heat. The diminished diameter 
of the tube was the inducement to pass the light but once, though it would have been 
possible to work with three passages. But when the rays skirt the walls of the tube, 
there is more disturbance from heat ; and, indeed, generally the advantage of 
augmented rotation is in great measure paid for by greater sensitiveness to deviation 
from optical uniformity. 
Not only does the communication of heat disturb the definition, but it tends also to 
render the actual temperature uncertain. During some of the more protracted sets of 
readings with the stronger currents there was a rise of nearly 2° in the temperature 
of the CS 2 ; and, although this rise was carefully watched, it is difficult to feel con¬ 
fident that the effective mean temperature can be determined with a less error than 
say ^ of a degree. Such an error would correspond to about -j oV o in the final number. 
To avoid increasing the uncertainty under this head the readings were often concluded, 
although the definition still remained satisfactory. 
If the apparatus were to be designed afresh I should endeavour to guard more 
adequately against these disturbances, and it might then be possible to use five 
passages with advantage, more especially if by increasing the weight of the coil it 
were practicable to bring the double rotation up to about 90°. The determination of 
such a rotation with the double image prism would be free in high degree from the 
polarimetric errors considered in § 10. But it is doubtful whether in the present state 
of science the additional accuracy would repay the labour involved. 
21. It only remains now to work out the results in absolute measure. And first as 
to the value of the standard current, defined as that which, flowing through the [l4] 
at 15°, balances Clark I. at the same temperature. This value in amperes is expressed 
by dividing the E.M.F. of Clark I. in B.A. volts (see Table XI. of former paper) by 
the resistance of the [1^] in B.A. units. Hence the standard current is 
