( 809 ) 
fore in the absolute number of amperes would, of necessity, occasion 
a relative difference twice as great between the values of the field 
obtained by the two methods (the same ammeter was used with the 
balance and with the solenoid). 
We have given the ballistic method a somewhat greater weight 
than the other on account of the smaller number of corrections it 
involved, and thus we have finally taken as the value of the 
standard field 
Once this standard field was definitely fixed all other measurements 
could be rapidly made by the ballistic method described above. 
For the conical pole-pieces which ate employed in experiments 
according to the maximum couple method, and which give much 
more powerful but much less uniform fields, we used a coil 
of 7 to 8 ram. diameter accurately centred on the axis of the 
pole-pieces. In this case direct comparison with the standard 
field just mentioned was not possible since the flat polepieces 
had to be screwed off to make room for the conical poles. 
To meet this case the area of the small coil encircled by the 
current was determined once and for all by withdrawing it from 
the standard field before the flat pole-pieces were removed, and 
comparing the change thus brought about in the number of the 
induction lines with those of the solenoid by means of the ballistic 
galvanometer. 
All the measurements that we have given up to the present refer 
to the field in the centre of the space between the poles. For the 
few exceptional values of the field, and, consequently, of the aseent 
of the liquid oxygen for which it. was necessary to cause it to rise 
pretty far above the axis of the pole-pieces, the field was determined 
at those points by simple ballistic comparison with the fields on the 
axis, ami we made use of the cathetometer to adjust the position of 
the small coil. 
54 * 
