682 
LORD RAYLEIGH OH THE VALUE OF THE BRITISH 
262'5 and the arc due to induction was 245'9. The difference 1*9 between 264*4 arid 
262'5 represented the defect of balance. In the second set of induction throws the 
corresponding difference is 1*3, showing that the changes of temperature in progress 
were (at this stage) improving the balance of resistances. The difference between the 
readings It and L with 853 units is 162'4, the reading L being the higher. Since 
the reading L is also higher with 753 units, we have to subtract from 162‘4 the mean 
of 1*9 and 1'3, i.e., 1’6. The corrected value is thus 160*8. With this we have to 
compare the mean of 246*6, 245*9, 245*7, 245*6, i.e., 245*9, and we thus obtain as the 
ratio of the two effects 
245-9 
160-8 
= 1*529 
The numbers obtained in this way were 1*535, 1*532, 1*529, 1*528, mean 1*5310 ; 
and with galvanometer reversed 1*534, 1*529, 1*530, 1*530, 1*532, mean 1*5310. The 
reversal of the galvanometer appears to have made no difference, and we have as the 
mean of all 1*5310. The comparison of the partial results shows that during the hour 
and a half over which the readings extended the battery current fell slowly about one 
part in 120, and that the resistance.of the copper gradually increased, until the balance 
was perfect, and afterwards became too great, the whole change being about one part 
in 6000, which would correspond to about one-twentieth of a degree centigrade. 
A small correction is required in identifying the above determined ratio with 
2 sin Ja/tan 9. If A be the induction arc and B be difference of equilibrium positions 
with 853 units when the commutator is reversed, 
jla in 
tan 2a=f^, tan 20=-^j 
where 
D= distance of mirror from scale=218 centims. 
From these we get 
2sin-|«_A ^ 32 4D 3 
tan 6 B x B 2 
l “4 
or in the present case with A=24*5, B=16*0, 
and 
2 sin \ c *. 
tan d 
=|(-99925) 
^=1-5310 
b 
So far we have omitted to consider the effect of damping, which must necessarily 
cause the observed value of A to be too small. If A. be the logarithmic decrement, 
the correcting factor is (1+X). The throw from zero to the first elongation is 
diminished by the fraction hX, and the distance from zero to the second elongation is 
