284 Profs. J. A. Fleming and J. Dewar. On the 



and were subsequently described to the Royal Institution in a lecture 

 delivered in 1892.* We have for some time past directed our attention 

 to the question of determining the numerical values of the magnetic 

 permeability and magnetic susceptibility of liquid oxygen, with the 

 object of determining not only the magnitude of these physical con- 

 stants, but also whether they vary with the magnetic force under which 

 they are determined. 



Although a large number of determinations have been made by 

 many observers of the magnetic susceptibility of different liquids 

 taken at various temperatures, difficulties of a particular kind occur 

 in dealing with liquid oxygen. One method adopted for determining 

 the magnetic susceptibility of a liquid is to observe the increase of 

 mutual induction of two conducting circuits suitably placed, first in 

 air, and then when the air is replaced by the liquid in question, the 

 susceptibility of which is to be determined. A second method con- 

 sists in determining the mechanical force acting on a known mass 

 of the liquid when placed in a non-uniform magnetic field. Owing 

 to the difficulty of preventing entirely the evaporation of liquid 

 oxygen, even when contained in a good vacuum vessel, and the 

 impossibility of sealing it up in a bulb or tube, and having regard 

 to the effect of the low temperature of the liquid in deforming by 

 contraction and altering the conducting power of coils of wire placed 

 in it, it was necessary to devise some method which should be indepen- 

 dent of the exact constancy in mass of the liquid gas operated upon, 

 and independent also of slight changes in the form of any coils of 

 wire which might be used in it. After many unsuccessful preliminary 

 experiments the method which was finally adopted as best complying 

 with the conditions introduced by the peculiar nature of the substance 

 operated upon is as follows : 



A small closed circuit transformer was constructed, the core of which 

 could be made to consist either of liquid oxygen or else immediately 

 changed to gaseous oxygen, having practically the same temperature. 

 This transformer consisted of two coils, the primary coil was made of 

 forty-seven turns of No. 12 S.W.G. wire, this wire was wound into 

 a spiral, having a rectangular shape, the rectangular turns having a 

 length of 8 cm. and a width of 1'8 cm. This rectangular-sectioned 

 spiral, consisting of one layer of wire of forty-seven turns, was bent 

 round a thin brass tube, 8 cm. long and 2-| cm. in diameter, so that 

 it formed a closed circular solenoid of one layer of wire. The wire 

 was formed of high conductivity copper, doubly insulated with cotton, 

 and each single turn or winding having a rectangular form. 



The turns of covered wire closely touched each other on the inner 

 circumference of the toroid, but on the external circumference were 



* See 'Roy. Inst. Proc.,' June 10th, 1892, "On the Magnetic Properties of 

 Liquid Oxygen." Friday evening discourse, by Professor J. Dewar, F.R.S. 



