312 Profs. J. A. Fleming and J. Dewar. 



The method used by us in these last experiments depends on the 

 well-known fact that if a body, either paramagnetic or diamagnetic, 

 is placed in a magnetic field of variable strength, it is subjected to a 

 mechanical force tending to displace it in the direction in which the 

 field varies most rapidly. 



If the susceptibility of the body is so small that it does not 

 sensibly disturb the distribution of the field, the measurement of this 

 mechanical force may be made to furnish a knowledge of the abso- 

 lute value of the magnetic susceptibility. 



The necessary conditions are, however, that the volume (V) of the 

 body must be of such small magnitude relatively to the form of the 

 field that its magnetisation is not sensibly different from that which 

 it would obtain if immersed in a uniform field, and also that the 

 magnetic susceptibility (&) of the substance must be of small absolute 

 value. Under these circumstances, if /is the mechanical force 

 (reckoned in dynes) acting ou the body, and H is the strength of 

 the field at its centre, then the force in the direction x is given by 

 the equation 



/=^YHf?. 

 ax 



The value k thus determined is a difference value, that is, it is 

 equal to the difference between the susceptibility of the body and 

 that of the medium in which it is immersed. Hence if one and 

 the same body is placed in the same divergent field, but alternately 

 surrounded by different media, the difference in the apparent suscep- 

 tibilities of the body in the two cases will give us the difference of 

 the true susceptibilities of the media. The experimental method em- 

 ployed by us consisted, therefore, in determining the forces acting on 

 a small sphere of known susceptibility when suspended first in air, 

 and next in liquid oxygen, and hence deducing the difference of the 

 susceptibility of liquid oxygen and air, and therefore the absolute 

 value of the susceptibility of liquid oxygen, knowing that of air. 



The first step was the construction of an electro-magnet capable of 

 producing the required field. From the above-named conditions of 

 success it will be seen that since the volume and susceptibility to be 

 measured are both small, it is essential that the magnetic field shall 

 not only be large but must vary very rapidly, or else the forces to be 

 measured will be small. 



An electro-magnet giving the required field was therefore designed 

 as follows : 



The exciting bobbin consists of a single coil of double cotton- 

 covered copper wire, No. 14 S.W.G. in size. 



The coil is 30 cm. long, 18'5 cm. in outside diameter, and the 

 aperture in the coil 9'5 cm. in diameter. The total weight of 



