217 



§ 2. The ab.b'ohite manometer. From the different tbnns described 

 by Knuüsen we chose one without sealed on 

 £ windows or ground joints in order that tlie iip[)a- 

 i-atus conld be heated to 300°. Tins heating is 

 advisable for the purpose of removing the occluded 

 gases fi'om the metallic parts and so obtaining a 

 P [)ernianent high vacuum. The working part of 

 the manometer is represented in fig. 1. A german- 

 f^ silver tube B of 'i nun. wall thickness was 

 Oi flattened by means of a roller and pi-ovided with 

 M a loose lid D, while the lower end remained 

 open. F, and F, are the two windows through 

 which the molecules enter the tube : thej proceed 

 from the wall of the glass vessel in which the 

 tube is contained and which is alternately at a 

 high and at the ordinary temperature, and strike 

 against the mica-plate M, movable round a vertical 

 axis, on which they exert a turning moment 

 which at a given temperature is proportional to 

 the pressure. The plate M is suspended from a 

 platinum strip (obtained by rolling out a platinum 

 wire of 0.08 mm.) and is provided with a piece 

 of iron (length 2 mm., diameter 0.07 mm.) for the 

 purpose of regulating the sensibility by means of 

 a magnet. The platinum strip is hard-soldered to 

 thicker platinum wire at both ends. The lower 

 wire is connected to M, the upper one passes through the screw E 

 and is clamped with the screw e. By means of E M can be made 

 exactly parallel to the flat sides of B. 



i4 is a platinum wire which prevents the mica plate falling down 

 when the apparatus is inverted. 



The two windows 0^ and 0^ serve for the reading of the rotation 

 of M : for this purpose the outer edge of the mica plate is observed 

 in a JjEitz reading microscope with eye-micrometer. The temperature 

 of the german-silver tube is read on a thermometer which is in 

 metallic connection with it and protected from i-adiation by means 

 of the tube H. 



The tube B is held in the glass vessel (fig. 2j by the springs S. 

 The dimensions of the various [)arts of the ap[)aratus were as follows : 

 area of V, 9.030. J. 18 cm^ 



„ „ F, 9.005 . 1 .20 cm^ 



10 



Fig. 1, 



20 



