PRELIMINARY NOTE ON A MAGNETIC PROOF PLANE 8? 



a hole in. in diameter through them for experiments on the rotation 

 of the plane of polarization of light, but which in these experiments 

 were closed by the solid poles which were screwed on. The first experi- 

 ments were with two discs of iron, 4*6 in. in diameter and If 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 magnetic field obtained at different points close to 

 the surface of the disc. These may be numbered as follows : No. 1, at 

 centre of face of disc; No. 2, on face of disc half an inch from the edge; 

 No. 3, on centre of edge of disc. The measures are on the metre, gram, 

 second system. 



1st. Strength of current, 4-4 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 current sent 

 through both magnets, so as to make the poles of the same name. 

 Current 4 '6 farads per second. 



1st. Distance of poles, 3 in. 



1. 1300. 3. 3800. 



2nd. Distance of poles, 1^ in. 



1. 600. 3. 4000. 



Here we see an approach 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. 



1st. Distance of poles, 3 in. 



1. 5800. 2. 8200. 3. 6700. 



2nd. Distance of poles, 1 in. 



1. 9800. 2. 7500. 3. 5800. 



It is curious to note how the distribution changes with the distance 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 3 in. apart, position 2 gives the greatest effect, 

 while, when they are 1 in. apart, the field is greatest at the centre. 

 This entirely agrees with theory. 



The conical poles for diamagnetic experiments were then screwed on. 

 These were portions of cones with an angle at vertex of about 60, with 

 the vertex considerably rounded off. They were one inch apart and 

 the poles were opposite. Current 4-4 farads per second. 



