100 MESSRS. J. HOPKINSON AND E. WILSON ON THE 
The attempt to obtain an approximation to the cyclic curve of induction from the 
curves in fig. 4 was a failure, that is to say, the resulting curve did not resemble a 
cyclic curve of magnetization. This is due to imperfections of fit of the two faces, in 
one of which the exploring coils are imbedded. That this imperfection of fit will tend 
to have a serious effect upon the distribution of induction over the whole area is 
obvious on consideration. Take the closed curve abcd in fig. 5, where AB is the 
junction between the pole pieces. If the space between the faces was appreciable, the 
force along be and ad in the iron could be neglected in comparison with the forces in 
the non-magnetic spaces ab, cd. The magnetizing force is sensibly 4zc, where ¢ is the 
current passing through the closed curve. This may be made as small as we please. 
Therefore, the force along ab is equal to the force along de. In our case the space 
between the faces is very small, but has still a tendency towards an equalizing of 
the induction per unit area over the whole surface. 
To test this the following experiment was tried. At a distance of 24 inches from 
the abutting surfaces of the pole pieces four holes were drilled in one of the pole 
pieces in a plane parallel with the abutting surfaces, as shown in fig. 6. By means of 
a hooked wire we were able to thread an insulated copper wire through these holes, 
so as to enclose only the square area A, which is bounded by the drilled holes and has 
an area of ‘61 sq. inch. The wire is indicated by the dotted lines. Fig. 7 gives two 
curves taken by the D’Arsonval in the manner already described for a reversal of the 
same current in the copper coils of the magnets. No. 1 (fig. 7) is the curve obtained 
from No. 1 coil (fig. 2) near the air space. No. 2 (fig. 7) is the curve obtained from 
the square coil shown in fig. 6. The difference is very marked and shows at once the 
effect of the small non-magnetic space which accounts for the large initial change of 
induction previously observed on the coils 1, 2, 3 in fig. 4. Similar holes were drilled 
in the yoke of the magnet in a plane midway between the vertical cores, having the 
same area of ‘61 sq. inch; and on trial exactly the same form of curve was produced 
as is shown in No. 2 of fig. 7. This method of drilling holes in the mass is open to 
the objection that the form of the area is square. 
Whilst the above experiments were being made the portion of the magnet to take 
the place of the pole-pieces previously used was being constructed as follows :—In 
fig. 8 the portion of the magnetic circuit resting upon the vertical cores consists of a 
centre rod A, of very soft Whitworth steel surrounded by tubes Ag, Ag of the same 
material. The diameter of A, is 1 mch. ‘The outside diameter of A, is 25 inches; 
and A, is 4 inches outside diameter between the cores of the magnet, but is 4 inches 
square at each end where it rests upon the magnet limbs. At the centre of the rod 
A, (longitudinally) a circular groove is turned down 1 millim. deep and 5 millims. 
wide, and also a longitudinal groove 1 millim. deep and 1 millim. wide is cut as 
shown in the figure for the purpose of leading a double silk covered copper wire from 
terminal T, to 9 convolutions at the centre and along the rod to terminal T,. A 
similar groove is cut in the outside of the tube Ay, and a copper wire is carried from 
