530 



PROFESSOR KNOTT ON THE STRAINS PRODUCED IN 



before this precaution was systematically taken, considerable trouble arose from the 

 occasional sticking of the meniscus in the capillary. 



§ 4. The Temperature Effect. — When the metal tube and part of the attached 

 capillary were filled with water, the whole acted as a very delicate water thermometer. 

 The change of temperature produced by the heating of the magnetising coils caused the 

 meniscus to race across the field of view of the microscope. This motion was, however, 

 almost entirely got rid of by adopting Professor Tait's device. A glass plug was made 

 so as to fill almost entirely the bore of the metal tube. By this means the water 

 in the tube and capillary was reduced so much in bulk that the temperature effect on it 

 was practically eliminated. On the other hand, any effect due to change of volume 

 of the tube was unaffected. It was found necessary, however, to set up the apparatus 

 the preceding afternoon, so that the tube and its contents were reduced to a steady 

 temperature throughout. While thus resting overnight, the end of the capillary dipped 

 in a shallow vessel of water, and was in this way kept quite full until the time of experi- 

 ment. When the experiment was to be made, the shallow vessel was removed, and 

 the drop at the end of the capillary absorbed by a piece of blotting paper. A gentle 

 downward pressure of the capillary through the rubber washer in the cap pushed out 

 a little of the contained water, which the blotting paper also absorbed. On the 

 pressure being withdrawn, the water in the capillary retreated along its narrow channel, 

 and in this way a meniscus could be formed in any convenient position in the capillary. 



The motion of the meniscus was generally observed through a microscope with a 

 micrometer scale. In the case of the nickel tubes in high fields, the displacements were 

 so great as to be most easily measured by eye. 



Every capillary used was calibrated, and the observed displacements of the meniscus 

 are all reduced to cubic centimetres. An attempt to increase the sensitiveness of the 

 apparatus by using an exceedingly fine capillary was a complete failure. The effect of 

 viscosity was then so pronounced as distinctly to hinder the motion of the meniscus. 

 The observations here recorded were obtained chiefly with two capillaries, whose cross 

 sections were respectively "00132 and "00139 square centimetres. 



§ 5. The Dimensions of the Various Tubes are given in the following Tables : — 



The Iron Tubes. 



No. 



Length. 



External 



Internal 



Volume of 



Diameter. 



Diameter. 



Bore. 



I. 



46"2 cm. 



3-84 cm. 



•635 cm. 



18-81 C.c. 



II. 







•953 



32-4 



III. 







• • . 



1-270 



60-16 



IV. 









1-588 



94-29 



V. 









1-905 



130-92 



VI. 









2-222 



169-76 



VII. 









2-696 



252-08 



3 









1-270 



67-7 



5 









1-905 



127-4 



7 







... 



2-540 



223-4 



