C,90 



For an object in the shape of a rod of uniform section s, the axis 

 of which is in the plane of sjmmetry of the poles and passes through 

 the middle of the interferrum, the relation is 



if H" and H' are the values of the field-strength at the ends of the 

 rod. When dealing with bodies of small dimensions by the method of 



Faraday, the spherical object is placed where H--~, therefore the 



dy 



force is a maximum. This is the method of procedure specially used 



by Curie in his classical researches. 



The rod-method, though applied long ago for measuring the suscep- 

 tibility of liquids by Quincke's method, was hardly used at all in 

 investigations on solids until 1910, when Pascal adopted it in his 

 important series of magneto-chemical researches '). 



This is certainly curious, as the principle of the method is very 

 simple and direct, but even more so as the disposition itself offers 

 imporiant advantages over the other methods. If one end of the rod 

 is placed in the middle of the interferrum and care is taken that 

 the other end is as far removed tVom it as possible, H" obtains a 

 maximum value and H' remains a quantity which may be neglected 

 or need only be taken into account as a correction. 



The susceptibility is thus given by a single field-strength which 



is much more easily determined than the product H —, which has 



to be derived from several values of H, not to mention the fact, 

 that the measurement itself of H in the middle of the interferrum, 

 where the field is most nearly uniform, can be carried out much 

 more accurately than at the point where the field is least uniform. 

 An absolute measurement by this method can therefore lead to a 

 much more trustworthy result. Moreover in using a rod a much 

 higher sensibility can be obtained, on the one hand because a larger 

 quantity of the substance can be utilized, on the other hand because 

 the intensity of the field in the middle of the interferrum can be 

 raised to a much higher \alue without any objection, which is not 

 by any means the case in the other method. Finally, as the field 

 near the middle of the interferrum can usually be made approxi- 

 mately homogeneous over a space of 1 cc, it is of no great import- 

 ance at what point exactly within that space the end of the rod 



1) P. Pascal, C R. 150, p. 1054. 1910. The priority of this application belongs 

 to GouY. G.R. 109, p. 935. 1889. 



