1881.] Determination of the Ohm in Absolute Measure. 133 



greater precautions were taken, in consequence of the experience thus 

 gained, to secure the box from the radiation of the lamp and gas jets, 

 which could not be dispensed with in the course of the experiments. 

 The magnet box was covered with o-old-leaf so as to reflect the heat as 

 much as possible. On the last night of work frequent determinations 

 of the position of rest were made, but in spite of all precautions an 

 unknown cause produced a sudden displacement of five scale divisions. 

 The exact time at which this change took place could not be deter- 

 mined, and two spinnings were therefore rejected. During the 

 remainder of the evening the magnet gradually came back to its 

 original position. With the exception of the two spinnings just 

 mentioned we have not rejected any observations. 



When we come to inquire into the amount of uncertainty to which 

 our results are liable, owing to the effects of these air currents, 

 we find that it cannot be greater than the more dangerous, because 

 less evident, errors to which the determination of our constants (mean 

 radius and distance of mirror from scale) are subject. As long as the 

 changes of the position of rest take place irregularly, the error would 

 tend to disappear in the mean, and the probable error deduced from 

 our experiments would give a fair idea of the uncertainty due to this 

 cause. This probable error, as we shall see, is very small. A regular 

 displacement of zero in one direction would, however, produce a con- 

 stant error which would not disappear in the final mean. We have 

 some evidence that such a regular displacement has to some extent 

 taken place. The comparison of zeros on December 6, as quoted 

 above, for instance, shows the position of rest in the course of the 

 spinnings shifted towards increasing numbers. Such a shift, if not 

 taken into account, would tend to make the deflections towards 

 increasing numbers (positive rotation) appear larger than those 

 towd'"^ decreasing numbers. This, indeed, was observed. Sur, posing 

 the shift takeb place regularly during the time of spinnings we might 

 have taken it i^o account. But the correction. -,/nich we should have 

 had to apply is 30 small and uncertain tnat it is doubtful whether we 

 should have imprbveu our final result, and it would certainly not have 

 altered it within the limits within which we consider it accurate ; for 

 we find that reducing the deflections on the supposition, 1st, that 

 the zero has kept constant ; and 2nd, that it has changed uniformly 

 during each set of spinnings ; the two results agree to within about 

 one and a-half tenths of a division, which, even at the lowest speeds, 

 would only make a difference of about 1 in 750, and on the highest 

 speeds four times less. The fact that a regular shift in the zero 

 position of the magnet affects the mean of four spinnings is due to 

 the arrangement of experiments, adopted during the first two nights, 

 in which four rotations succeeded each other in alternate directions. 

 If, after a rotation in the positive direction, two negative rotations, 



