283 



lead circuit closed. Altliougli the matter requires fiu-thei' elucidation, 

 it would seem to follow from this ineasiiremeut, that a lew of the 

 layers of the coil are sliort-circnited. After opening the circuit a 

 residual moment remained in the coil as l)efore which was destroyed 

 on raising the temperature slightly above that of the helium-bath. 



Physics. — "Farther experiments loitli Uqaid liellum. K. Appedrance 

 of beginning paramagnetic saturation." By Prof. H. Kamkri.ingh 

 Onnes. Communication N°. J 40(;/ from the Physical Laboratory 

 at Ley den. 



(Communicated in the meeting of May 30, 1914). 



The question, whether paramagnetic substances would show a 

 saturation-etFect at high tield-streugths, has always been considered 

 a very important one. Althongh it could hardly be assumed, that 

 the susceptibility would remain independent of the field at higher 

 strengths than were attainable, still so far at the highest fields 

 available it appeared to be the case. Lx\ngf-vin's theory brought 

 the explanation, why so far all attempts to find paramagnetic satu- 

 ration-effects had remained unsuccessful. According to this theory 

 the magnetisation appears to be determined by the expression 



6,n H 



a = -, where 'J„, is the magnetic moment of the molecules per 



RT 



gramme-molecule, R the gas-constant, T the absolute temperature 

 and- H the field. As long as a remains below 0,75, the changes of 

 the susceptibility with the field escape the ordinary method of obser- 

 vation and at the ordinary temperature even a substance as strongly 

 paramagnetic as oxygen gives for a with a field of 100000 not moi-e 

 than about 0,05. As I pointed out at the 2"'^ International Congress 

 of Refrigeration at Vienna (1910) this theory shows that lowering 

 the temperature is the means by which the observation of para- 

 magnetic saturation might be attained and that helium-temperatures 

 are the most suitable for the purpose. In fact as the absolute tem- 

 peratures to which one may descend by means of helium are 70 

 and even 150 times lower than the normal temperature, the result 

 will be equi\alent to raising the magnetic field at which the obser- 

 vation is made 70 or J50 fold. 



T have lately at last been able ^) to fulfil my desire to attack by 



1) Viz. by the acquisition a short time ago of an electromagnet (built according 

 to Wtiss's principle and utilising his friendly advice) the interfcrrum of which 

 leaves sufficient room at fields of 20000 for experiments with liquid helium. 



