698 



distance of 1 cm. The field-strengths were measured with a CottoN- 

 balance of tiie nana! pattern by W. C. Wkbeh of Zurich. 



The circuits of tlie fixed and movable coils are entirely independ- 

 ent of each other : each of them contains an accurate ammeter, a 

 commutator and rheostats, in which the current is reversed on com- 

 mutation, in order to neutralise any magnetic influence on the ammeters. 

 They are within reach of the observer seated in front of the microscope. 



The field-strenglh of the electi'omagnet is given by the current 

 flowing through it; the field was not adjusted until the magnetising 

 current had been^seveial times reversed. 



If the evaporation of the bath in the apparatus as described is 

 too strong, as is the case when liquid hydrogen is used, it is dimi- 

 nished by surrounding the bottom half of the vacuum-glass with a 

 vacuum-glass with liquid air. 



§ 6. Method of observation. Passing by certain simplifications 

 which were often possible we proceeded as follows. 



The enclosure and the carrier (without experimental tube) are 

 first adjusted so that the common axis is vertical and passes through 

 the centre of the interferrum. When lliis position is arrived at, the 

 apparatus is not moved sideways any more. 



The experimental tube is then screwed to the carrier and its 

 spring clamped. By means of the weight on scale M^ the carrier is 

 made to float on the mercury approximately at the desired level 

 and care is taken that the movable coil has the correct position 

 relatively to the fixed coil. The apparatus as a whole is then moved 

 in a vertical direction until the lower end of the experimental cylinder 

 falls about in the line of the axis of the poles, after which the 

 cyhnder is adjusted more accurately by turning the apparatus about 

 the ball socket 6\. When finally the poles have been brought at the 

 right distance, everything is ready for the observations at ordinary 

 temperature. 



In changing to low temperatures as much weight is added to the 

 scale as agrees approximately with the upward pressure of the bath 

 to be expected and the cover is fastened to the apparatus air-tight 

 by means of the india-rubber ring ; after drawing out the poles, the 

 vacuum-glass is placed carefully round the experimental tube, con- 

 nected airtight and centred in a manner similar to that used in the 

 apparatus of Weiss and Kamerlingh Onnes. After having made sure 

 that everything is airtight, the liquid gas is admitted to the vacuum- 

 glass, the poles are brought back to their position, and the carrier 

 is adjusted to its zero by means of the plunger; the currents in the 



