PARAMAGNETIC & DIAMAGNETIC SUBSTANCES 283 

 fibre supporting the torsion rod was attached to a graduated head, 

 and the position of the rod was read by a mirror and scale. Its zero 

 position was determined by removing the cylinder and placing an 

 equal load on the rod itself, where any force due to the field was 

 negligible. This zero position was so 

 arranged that the vertical through the 

 end of the rod passed through the point 

 m, 0'5 in. from c. Then the body was 

 suspended, and it was forced inwards 

 or outwards according as it was more 

 or less paramagnetic than the sur- 

 rounding medium, the torsion rod 

 being pulled round one way or the 



other. The torsion head was then turned round till the rod was 

 again in the zero position and the angle of torsion measured the 

 force acting. Since the force due to the field was exceedingly 

 small compared with the weight of the body, the suspending fibre 

 was >o near the vertical that the body could be regarded as at the 

 point vertically under the end of the arm. 



The standard force selected was that on water in air, and was 

 taken as 100. To determine it a cylinder of glass was suspended 

 in air. It was repelled from the zero position, and required 15 of 

 torsion to restore it. It was then suspended in water. Now it was 

 attracted and required 54*5 of torsion to restore it. If /C K , /c a , /c vv 

 are the susceptibilities of glass, air, and water respectively, 



FIG. 217. 



K K K w oc + 54'5 

 whence K W /c a oc 69*5. 



Then 69'5 of torsion are represented by 100 on the scale 

 chosen. 



Now. Mipposc that the same cylinder is hung in any other liquid 

 with Mix-rptibility K V and that 6 of torsion are required to 

 nature it. 



Then K S K a: 0. 



But K s AC a OC 15. 



Subtracting KI - /c a x - (15 + 6) 

 and this is expressed on the air-water scale by 



If the susceptibility /c s of a solid was required, taking a cylinder 



