689 



Physics. — "Max/Witte reHeardtes. X. Apparatus for tlui (jenend 

 cnjomagnetlc investigation of substances of small suscepiihiUti/." 

 By Prof. H. Kamerlingh Onnes av.d Dr. Albert Perrier. 

 Communication N". 139r? from the Physical Laboratory at 

 Leiden. (Communicated by Prof. H. Kamerlingh Onnes). 

 (Communicated in the meeting of October 25, 1913). 



§ 1. Introduction. This paper contains tlie full description of the 

 apparatus used in the investigation of Communication III (Comm. 

 N". 122a, continued in IV, Coram. N". 124^?) of this series (Proceed- 

 ings of May 1911). Various circumstances have retarded the extensive 

 description which was pi-omised there instead of the rough sketch. 



The construction of the apparatus to be described forms part of a 

 more general scheme to gradually obtain the necessary appliances 

 for the investigation of weak magnetisation at low temperature. In 

 doing this we did not contine our attention to special measurements, 

 but intended to enlarge with as many appliances as possible the 

 almost completely unknown "technique" of investigations in this field. 



On the one hand the measurement of magnetic forces, on the 

 other that of magnetic couples suggest themselves. The ballistic 

 method (measurement of flux) is only applied in the study of ferro- 

 magnetism ^). 



The method of couples is speciallj' suitable for crystals and for 

 isotropic bodies, which by their shape are seemingly magnetically 

 anisotropic (e. g. ellipsoids). The apparatus with which our first 

 measurements on the susceptibility of liquid and solid oxygen were 

 made (Comm. N". 116, Proceedings April 19J0) is based on this method.-) 

 In a modified form this piece of apparatus will, we hope, be soon 

 utilized in the cryomagnetic investigation of crystals. 



In measuring forces a non-homogeneous field is used. Two cases 

 have to be distinguished here. 



For an object of small dimensions (the volume of which is v and 



the volume-susceptibility K) placed in the plane of symmetry between 



the poles of a magnet the force in the direction of the middle of 



the interferrum is given by 



dB 

 F—vKH-- 



dy 



where H indicates the intensity of the field and y the coordinate at 

 right angles to the field. 



^) In some cases wliich we will not dwell upon here this method might be 

 resorted to. 



'-) The apparatus used by Weiss and Kamerlingh Onnes for the investigation of 

 lerromagnetisra at low temperature (Comm. No. 114) belongs to the same type. 



