Susceptibility of Iron in Colloidal Solution. 



261 



Apparatus. 



On account of the extreme tenuity of the colloidal solution 

 and the small quantity available, it was necessary to be able to 

 make delicate measurements on small quantities of the solution. 

 The method adopted was that first suggested by Lord Kelvin* 

 and afterwards used by Wills "f" in his experiments on the sus- 

 ceptibility of weakly magnetic solids and liquids. 



A general idea of the apparatus is given in Fig. 1. A rect- 



Fig. 1. 



angular slab (A) of the material whose susceptibility is to be 

 measured, is so suspended from one arm of a balance (B), that its 

 long axis is vertical and its lower face is in the strongest part of 

 the field between the poles of an electromagnet. 



If k = susceptibility of the material, 



A = area of the horizontal section of slab, 

 H l = magnetic field at bottom of slab, 

 H. 2 = magnetic field at top of slab, 



then the pull, P, on the slab due to the magnetic field is given by 

 the equation 



kA{H?-Hi) 



P = 



*9 



grams. 



In the present experiment the pole pieces were prismatic ones, 

 whose vertical faces were 7*5 cms. by 1 cm., the longer edges being 



* Brit. Assoc. Rep. 1890, p. 745. 



t Wills, Phys. Rev., April, 1898; ib. March, 1905. 



