PARAMAGNETIC & DIAMAGNETIC SUBSTANCES 285 



where A and B are known if we know the rates of change of field 

 along and perpendicular to the axis. These rates were determined 

 by means of an <; exploring coil " which was connected to a ballistic 

 galvanometer and placed, to begin with, at the centre of the field and 

 with its plane perpendicular to the field. It was then moved short 

 measured distances and the galvanometer throws gave the changes 

 to be measured. The coil was also moved from the centre right 

 out of the field and thus the field at the centre was found. 



The method was applied to crystals of bismuth and of calc 

 spar, and it was found that the value of AC in each case was different 

 along different axes. The values of 10 9 /c obtained along two axes 

 were 



BISMUTH CALC SPAR 



lOVi - 12554 - 38 



10V, - 14324 - 40 



These appear to be the earliest determinations in absolute 

 measure. 



Experiments of von Ettingshausen. Four different 

 methods were used by von Ettingshausen * to determine the sus- 

 ceptibility of bismuth. In the first method two primary coils 

 were placed in series and an intermittent current was sent through 

 them. Round the primary coils were wound secondary coils con- 

 nected in series through a galvanometer but in opposition that 

 is, so that the simultaneous currents induced in them by the 

 primaries went in opposite directions through the galvanometer. 

 There was a commutator in the circuit worked at such a rate 

 that both make and break currents went through the galvanometer 

 in the same direction. First the coils were so adjusted that the 

 secondary currents just neutralised each other. Then a bismuth 

 cylinder was inserted in one of the primaries. Since the per- 

 meability of bismuth is less than unity, the mutual induction 

 between that primary and its secondary was decreased, the current 

 induced in it was less than the current in the other secondary, 

 and the galvanometer was deflected. The bismuth was then 

 withdrawn, and in its place was inserted a solenoid through which 

 a known current was passed, made and broken as frequently as 

 the primary current in the previous experiment. This induced 

 an alternating current in the secondary, rectified as before through 

 the galvanometer. The magnetisation of the bismuth under the 

 original known primary could thus be compared with the mag- 

 netisation if we may so term it of the inserted solenoid with 

 its known current, and the susceptibility of the bismuth could be 

 determined. 



* Wied. Ann., xvii. p. 272 (1882). 



