a Dielectric in an Electrostatic Field. 203 



Weight 1000 grams. 

 Wt. put oil. Wt. taken off. 



28-7 31-2 



39-2 29-4 



30-3 27-7 



:)2'0 29-6 



32-0 



The first column gives the absolute elongation ; the second 

 indicates the gradual change of the zero. The mean deflexion 

 is about double for the increased weight. The modulus of 

 elasticity, which should be about 3x 10 n , is determined from 

 the following data : — 



Inner diam. of glass tube . . . 5'80 cm. 



Wall thickness ...".. 0'25 cm. 



Cross-section 4*25 cm. 



Length 55*00 cm. 



Weight 1000 grams. 



Change of length 32*0 div. 



Value of one division .... 1*5 X 10 — 6 cm. 



E (Young's modulus) . . . . 2- 7 x 10 11 



From the value of Young's modulus obtained, it is evident 

 that the mirror recorded changes of length approximately 

 correct. The three cylinders were then adjusted, the inter- 

 vening spaces filled with a non-conducting fluid and the outer 

 one charged. The series of experiments described are from 

 a very large number of attempts extending over more than 

 two years. 



The etherial salts of acetic acid are liquids of approximately 

 the same specific inductive capacity as glass, which was the 

 solid used. But amyl acetate, having about the same resistance 

 as the others, did not hold a sufficiently high charge. I then 

 decided to try kerosine, which though of* a less inductive 

 capacity, yet would reduce the surface charge on the glass 

 by half. With this liquid sparks of two centimetres were 

 gotten. In spite of great care, I was troubled by the fact 

 that when the outer cylinder was charged the glass was 

 bent laterally. This lateral bending was easily recognizable, 

 as it caused the field to move laterally and at the same time 

 either up or down. By placing the tilting mirror on opposite 

 sides of the rim of the tube opposite effects would be observed. 

 The lateral motion was accompanied at times by an up or 

 down motion of ten or twenty divisions of the micrometer. 

 Occasionally, for the apparatus was adjusted many times, the 

 lateral effect was rendered quite small ; a few times eliminated, 



