of Edinhurgli, Session 1883-84. 559 
represents the same conditions as those of the experiment, probably 
because of the neo-lected effect of ”1. 
dyj 
With a frame constructed as described, the error due to any small 
excentric position of tlie magnetometer is insensible. Practically, 
however, there will be some difference of thickness in the front and 
rear coils due to imperfect workmanship. We shall make a liberal 
allowance by supposing that one coil is broader than the other by 
Value of c in centimeters. 
Fig. 11. 
1 mm. Also let the magnetometer be placed 1 mm. east or west of 
the centre of the frame. The value of the deviating force will then 
be the value of the ordinate at c= 10 in curve II., fig. 8, multiplied 
by in x 0.1 x 0. 1, which is 0.0042 x 0.01 x in = 0.000042 in. This, 
divided by F,, the force along the axis NS, will give the angle e 
(page 551). 
To find in the assumed conditions of excentricity : — 
Let w be the solid angle subtended at m by a thin shell vLose 
thickness is 8c, and n= the number of turns per unit length. Then, 
using the same notation as before, 
TT-l . d(li . 
