76 HENRY A. EOWLAND 



piece h. Into these pieces are screwed two tubes c made of iron and 

 having an internal diameter of about T 7 T in. and a thickness not to 

 exceed ^ in. Through these tubes the iron rods a I slide and are 

 held at any point by the screws d. One end b of this rod is rounded 

 off for diamagnetic experiments and the other enlarged and flattened 

 at the end for magnecrystallic experiments. On the tube c a helix of 

 N~o. 16 or No. 18 wire is wound so as to make up a thickness of -4 or -5 

 of an inch and having a length of 2 in. The object of the screws g is 

 principally to allow the rods a & to be reversed quickly and to adjust the 

 position of the helices. When the apparatus is to be used for only one 

 kind of work it can be much simplified by doing away with many of the 

 moving parts. 



This instrument can be used either with the ordinary magic lantern, 

 or better, with one having, a vertical attachment. In the latter case 

 the plane of the instrument is horizontal and the substances are sus- 

 pended from a wire made quite small, so as not to cut off too much 

 light. 



The suspending thread in the case of bismuth can be quite large 

 but for other bodies a single fibre of silk is best; these in the shape of 

 bars half an inch long can be each attached to a fibre having a little 

 wire hook at its upper end and hung in a cabinet until required. 



The theory of feebly magnetic or diamagnetic bodies oscillating in 

 a magnetic field is very simple and yet the results are of the greatest 

 interest, especially the effect of the size of the apparatus, which is 

 here given for the first time. 



2. THEORY 



Let a very small particle of a body whose coefficient of magnetization 

 AC is very small, and either positive or negative, be placed in a magnetic 

 field of intensity R; it will then have an induced magnetic moment of 

 <vR, where v is the volume of the element. The force acting on this 

 particle to cause it to go in any given direction will be equal to the 

 product of the magnetic moment into the rate of variation of R in that 



direction, 1 and hence is K vR ~r in the direction of x. The total force 



ax 



acting on the body in the direction of x is therefore 



1 Thomson, Reprint of Papers, art. 679, Prob. vii. 



