16. H. A. Rowland—Magnetie Proof Plane. 
coil and Q is that component of the magnetic field we are 
measuring in the direction of the axis of the small coil. 
As an illustration of this method I will give a few experi- 
ments made with the magnets of a Ruhmkorff diamagnetic 
apparatus, which was altogether about 2 ft. long and had its. 
magnets 2 in. in diameter, with a hole 4 in. in diameter through 
them for experiments on the rotation of the plane of polariza- 
tion of light, but which in these experiments were closed by 
the solid poles which were screwed on. The first experiments 
were with two dises of iron, 4°6 in. in diameter and 1@ in. thick, 
screwed on to the poles. In the first place the poles were 
turned away from one another, the current being sent through 
only one magnet, and the values of the ee field obtained 
at different points close to the surface of the These may 
be numbered as follows: No. 1, at center of fate of disc; No. 
2, on face of disc half an inch from the edge; No. 3, on cen- 
ter of edge of disc. The measures are on the meter, gram, 
second system. 
lst. Strength of current, 44 farads per second. 
1. 2220. 2. 3550. 3. 4440. 
2nd. Strength of current 8°3 farads per second. 
1. 3600. 2. 5300. 3. 7500. 
Next the poles were turned toward each other and the cur- 
rent sent through both magnets, so as to make the poles of the 
same name. Current 4:6 farads per secon 
1st. Distance of poles, 3 in. 
1. 1800. 3 3800. 
2nd. Distance of poles, 14 in. 
1. 600. 4000. 
Here we see an apoteath to one of Faraday’s places of no 
magnetic action 
After this the current in one of the magnets was reversed so 
as to make the poles opposite. Current the same. 
Ist. Distance of poles, 3 in. 
1. 5800. 2. 8200. 3. 6700. 
2nd. Pager of poles, oe in. 
9800. 7500 3. 
It is curious to note how she distribution changes with the dis- 
tance of the discs; thus, on one disc free from the other, the 
edge of the disc has the greatest magnetic surface density, but 
when the two discs form opposite poles and are 8 in. apart, 
tion 2 gives the greatest effect, while, when they are digs 
apart, the field is greatest at the center. This entirely agrees 
with theory. 
