( 806 ) 
for details regarding its construction. It was only necessary to replace 
the conical pole-pieces by cylinders with flat ends. Their distance 
apart was micrometrically adjusted to 25 mm. and controlled with 
an accurate callipers. We may here remark that between the measu¬ 
rement of the ascent and that of the field, the pole-pieces remained 
clamped tight to the cores, so the adjustment of the distance could 
give rise to no error. 
Since in the subsequent calculation the strength of the field is 
involved to the second power, and since we are concerned with an 
absolute measurement, we endeavoured to make our measurement 
of the field strength as trustworthy as possible with our present 
appliances. With this end in view we measured the strength of 
an arbitrarily chosen standard field by two different processes, and 
we compared the strengths of the fields used in our experiments 
with this standard by successively withdrawing the same coil 
attached to a ballistic galvanometer from the standard field and from 
the various fields which we desired to measure. 
The standard field was set up with the same flat pole-pieces at 
a distance of 9 mm. apart, and with a current of 5 amp. All pre¬ 
cautions were taken to ensure the demagnetization of the magnetic 
cycle beforehand. This field was first measured by means of 
Cotton’s magnetic balance 1 ). As is well known this method consists 
of equilibrating weights of a total mass m against the ponde- 
romotive power of the field H on a straight portion of length l 
of a conductor through which a current flows of intensity i; then 
we get 
For the degree of accuracy, however, which we wish to reach, 
several corrections must be taken into account. In the first place 
the various parts of the balance were accurately calibrated. The 
length l of the current element was determined micrometrically and 
on the dividing engine, and so also was the distance between the arcs 
of the balance which distance ought to be the same throughout 
seeing that the arcs must be accurately concentric. The very small 
deviations from this were allowed for by means of a ballistic 
investigation of the topography of the field. The balance arms of 
the weights and of the current element were measured with the 
cathetometer. The topographical study of the field also gave us the 
For this method of measuring the field and for the magnetic balance see: 
P. Weiss and A. Cotton, Le phenomene de Zeeman pour les trois raies bleues 
da zinc, Bull. Seances Soc. franq. de phys. 1907, p. 140, also J. de phys. 1907. 
