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



through the primary circuit of the small transformer is increased in 

 the same proportion that the permeability of the transformer core 

 is increased by the substitution of liquid oxygen for gaseous oxygen r 

 and hence the ballistic deflection measures at once the amount by 

 which the magnetic permeability of the liquid oxygen is in excess 

 over that of the air or gaseous oxygen forming the core of the trans- 

 former when the transformer is lifted out of the liquid. As a matter 

 of fact it was nfiver necessary to obtain the inductive balance pre- 

 cisely. All that was necessary was to observe the throw of the bal- 

 listic galvanometer, first when the transformer was wholly immersed 

 under the surface of liquid oxygen, and, secondly, when it was lifted 

 out into the gaseous oxygen lying on the surface of the liquid, the 

 strength of the primary current reversed being in each case the 

 same. In order to standardise the galvanometer and to interpret the 

 meaning of the ballistic throw, it was necessary to cut out of circuit 

 the primary coil of the balancing induction coil, and to reverse 

 through the primary circuit of the small transformer a known small 

 primary current, noting at the same time the ballistic throw pro- 

 duced on the ballistic galvanometer, this being done when the 

 transformer was underneath the surface of liquid oxygen. It will 

 be seen, therefore, that this method requires no calculation of any 

 coefficient or mutual induction, neither does it involve any know- 

 ledge of the number of secondary turns on the transformer, nor of 

 the resistance of the secondary circuit ; all that is necessary for a 

 successful determination of the magnetic permeability of the liquid 

 oxygen is that the secondary circuit of the transformer should 

 remain practically of the same temperature during the time when 

 the throw of the ballistic galvanometer is being observed, both 

 with the transformer underneath the liquid oxygen and out of the 

 liquid oxygen. If then the result of reversing a current of A 

 amperes through the two primary coils in series when the secondary 

 coils are opposed is to give a ballistic throw, D, and if the result of 

 reversing a small current a amperes through the primary coil of the 

 transformer alone is to produce a ballistic throw, d, then if p is the 

 magnetic permeability of liquid oxygen, that of the gaseous oxygen 

 lying above the liquid and at the same temperature being taken as 

 unity, we have the following relation : 



"T = P- !> 



-d 

 a 



which determines the value of /JL. 



Deferring for a moment the correction to be applied to determine 

 the value of the magnetic permeability of liquid oxygen in terms of 

 that of a vacuum, the following are the results of observation : 



