ASSOCIATION UNIT OF RESISTANCE IN ABSOLUTE MEASURE. 
681 
acted upon by the induction impulse (in the opposite direction to that observed before), 
and the four following elongations were read. 
These observations of the throw were followed as quickly as possible by observations 
of the effect of substituting 853 for 753 units in the auxiliary arc. As soon as the 
vibrations could be reduced to a moderate amplitude, readings of three or four con¬ 
secutive elongations were taken. The galvanometer contact was then broken, and the 
battery key reversed. When the needle had swung over to the other side, the 
galvanometer contact was renewed, and four elongations were observed. The 
difference between the two positions of equilibrium represented the disturbance of 
the resistance balance. 
The whole of this disturbance, however, was not due to the additional 100 introduced, 
but required correction for the corresponding effect observed even with 753 units in 
the auxiliary arc. For this purpose it was only necessary to add or subtract the 
difference between the equilibrium positions of the needle with the key in the two 
positions, as deduced from the observations immediately preceding the induction 
throws ; and in order to eliminate the influence of the progressive change, the mean 
of these differences as found before and after the insertion of the extra 100 units was 
employed. This result was compared with the mean of the four induction throws 
contiguous to it, two preceding and two following, and in this way a ratio obtained 
which was independent of the gradual but unavoidable changes in the battery current 
and in the copper resistance. After about half the readings had been taken the 
galvanometer connexions were reversed. 
A specimen set of observations will now be given. 
3 h 36 m [753] 
L. 
264*4 
Induction. 
246*6 
3 h 38 m 
R. 
262*5 
3 h 38 m Induction ...... 
245*9 
3 h 40 m [853] 
R. 
182*3 
3 h 41 m 
L. 
344*7 
3 11 44 m [753] 
L. 
264*4 
3 h 44 m Induction. 
245*7 
3 11 45 m 
R. 
263*1 
Induction. 
245*6 
At 3 h 36 m with 753 units in the auxiliary arc and with battery key to the left, the 
position of equilibrium, as deduced from two elongations, was 264*4 on the galvano¬ 
meter scale. The arc of vibration due to induction consequent on shifting the key 
from left to right, corrected for the free swing, but uncorrected for damping, was 
246*6. In like manner with key to the right, the equilibrium position at 3 h 3S m was 
4 s 2 
