697 



It is desirable, (hat tlie ui>ward pressure due to tlic l;atli t-lianges 

 as little as possible wheji llie liquid level falls tlii-oiigli evaporation; 

 for this reason the tubes end at the t(>|) in tliiji glass rods M^ of 

 2to 2.5 mm. diameter. The lower spiral spring R., and the thread 

 by means of which the tube is screwed to the carrier are sealed to 

 this rod with some Kiiotinsky glue. As regards the shape of the tube 

 we have used different forms. Tubes as shown at 5, are used for 

 substances of high susceptibility, for which (he magnetic action on the 

 glass or on the bath plays a subordinate part, co that for them it may 

 be entirely neglected or else a correction may be easily applied. The 

 tube is filled with the substance, when it is still open, at the bottom 

 putting in small quantities at the time, which are evenly compressed 

 in order to obtain a tight filling and at (he same time a uniform 

 density throughout the whole tube; the substance is then closed in 

 with a small plug of glass wool to prevent its being heated during 

 the sealing of the tube and the tube is sealed ofT at the air-pump. 

 The smaller the susceptibility of the substance the greater influence 

 the susceptibility of the air would have and the morene cessary it is 

 to be assured of a good vacuum; a high vacuum, however, is 

 obviously unnecessar3\ 



In cases where account has to be taken of the susceptibility of 

 the glass, which may give rise to fairly strong forces '), tubes of 

 type S^ are used, the lower half of which, separated from the upper 

 half by a glass partition, is exhausted. When this partition is placed 

 on the level of the axis of the poles, the correction for the glass 

 disappears, as it is divided symmetrically with respect to the axis; 

 the susceptibility of the substance is in that case directly compared 

 with that of the vacuum. Type S^, which does not require further 

 explanation is meant for the measurement of the susceptibility of 

 the liquid in the bath. 



§ 5. Additional apparatus. The electromagnet is a copy of Weiss's 

 electromagnet which was used in previous reseaj-ches of this series. 

 The yoke is, however, placed hoi-izontally this time, in order to 

 leave the space below the apparatus compleiely free (comp. per- 

 spective drawing). Usuallj poles were used of the shape shown, 

 the flat end-faces of which had a diameler of 40 mms. At a polar 

 distance of 15 to 20 mms. the topographical inequality of the field 

 about the middle of the interferrum was not above 0.1 "/o within a 



Tl,e susceplibilily of glass at low temperature was delermined by us in 

 Comm. No. 124a, p. 6. 



