Magnetic Permeability of Liquid Oxygen and Liquid Air. 291 



of observations marked I, II, III, IV, the mean of the means of the 

 three observations in Sets I and III, corrected for the variation in 

 the primary current, were taken as the result of the measurement in 

 liquid oxygen, and this result was then compared with the ballistic- 

 throws in Set II. 



Again, the mean of the means of sets of observations II and IV,, 

 properly corrected for variation of primary current, were compared 

 with the mean of the observations in Set III, and the result is to give 

 the data for calculating the permeability of the liquid oxygen for a 

 primary current through the primary coil of the transformer of about 

 37 amperes, corresponding very nearly to a mean magnetising force 

 of 166 C.Gr.S. units. The sum or difference of these means of the 

 throws, taken in the liquid oxygen and out of the liquid oxygen, 

 depending on whether they are on the opposite or the same side of 

 the zero of the scale, gives us the value of the quantity denoted by 

 D in the Table I below, and in the formula for the value of /*. 



The above sets of observations, I, II, III, and IV, refer to a 

 primary current of about 37 amperes ; but similar sets of observa- 

 tions were taken with a primary current of about 8 amperes, 28 

 amperes, and 50 amperes respectively, and the result's of all these- 

 observations, which are included in the sets of observations, I to XII, 

 above given, have been reduced in Table I below to show the mag- 

 netic permeability of the liquid oxygen corresponding to different 

 megnetising currents. The set of observations marked Experiment 

 Valid Experiment VI in the above table of results, gives the observa- 

 tions for standardising the ballistic galvanometer. In the first case- 

 the primary coil of the balancing induction coil was cut out, and a 

 primary current, having a value of 0*1145 ampere, was reversed 

 through the primary coil of the transformer alone, and gave ballistic- 

 deflections as stated in the observations in Set V. These observations 

 serve to standardise the galvanometer and interpret the meaning of 

 the throw obtained when the large current is reversed through the 

 primaries of the two induction coils, the secondaries of which are 

 opposed. It will be noticed that one important advantage of the- 

 above- described method is that the quantity which we desired to 

 know, viz., the amount by which the presence of the liquid oxygen 

 increases the magnetic permeability of the core of the transformer, ia 

 the quantity which is measured directly, and that any error in the 

 measurement of this quantity does not affect the permeability to 

 anything like the same proportional extent. An error of about 10 

 per cent, in the measurement of the ballistic throw would only affect 

 the fourth place of decimals in the number representing the perme- 

 ability of the liquid oxygen. 



The results of all the above observations, when reduced, are com- 

 prised in the following table : 



VOL. LX, z 



