202 Dr. L. T. More on the Supposed Elongation of 



of the tilting mirror rested on the glass and the other two on 

 the brass tube, or vice versa. By screwing the cap up or 

 down a proper level of the mirror could be readity obtained. 



Finally the entire apparatus was rigidly fastened to a system 

 of heavy beams fitted on a brick pier in a basement room 

 where the temperature was reasonably constant. 



The inner tube was always connected with the ground both 

 at top and bottom, and since the metal legs rested on the head 

 of the tube, the silvered front and back surface of the mirror 

 was also uninsulated. A sheet of card-board covered with tin- 

 foil fitted closely to the top of the glass tube, and on being 

 grounded prevented any direct attraction between the mirror 

 and the charged electrode. The outer brass tube, or else a 

 covering of tinfoil on the outer surface of the glass tube, as 

 the case might be, was charged by a powerful electric machine 

 capable of giving a spark, of great volume, eight or more 

 inches in length. One pole of the machine was grounded. 

 A Lane unit jar, with a spark-micrometer of brass balls two 

 centimetres in diameter, was included in the circuit and one 

 ball grounded. The jar stood near the apparatus, that there 

 might be little difference of potential between the tubes. The 

 fall of potential along the entire line was so small that the 

 sparks passed as readily between the balls of the jar as be- 

 tween those of the machine. When they were set at the same 

 distances, sparks usually passed simultaneously. 



To test the sensibility of the apparatus several experiments 

 were made by determining the elasticity of the glass tube. A 

 table was prepared with three legs that rested upon the top of 

 the glass tube alone, on which weights could be laid. The 

 results of a set of readings are given below. As the only 

 object was to test the sensibility of the apparatus, the experi- 

 ment was performed without especial care to avoid changes of 

 temperature or other causes which affect the zero-point when 

 a sensitive arrangement is used. 



Deflexions in Divisions of Micrometer. 

 Wt. 500 grams. 



Wt. put on. 



Wt. taken off, 



213 



14-2 



16-7 



14-0 



16-25 



14-8 



16-65 



11-5 



13-4 



12-6 



16-86 



