4 Prof. L. T. More on Electrostrietion, 



the intervening spaces filled with a non-conducting fluid 

 having a dielectric constant equal to that of the solid dielectric, 

 Prof. Ercolini* made a series of experiments with such 

 armatures. 



The arrangement of his apparatus was similar to mine, 

 except that the changes of length were measured by the 

 displacement of interference-bands. 



A comparative table of the dimensions of the apparatus 

 will show their relative advantages. 



Table I. 





Ercolini. 



More. 



Former. 



Present. 



Outer diameter of glass tube (A) ... 

 Inner „ „ ,, „ ... 

 Thickness ,, „ „ ... 

 Distance between A and armatures . 



,. „ armatures 



Length of armatures 



16-33 mm. 



15-30 „ 

 0-52 „ 

 2-00 „ 

 4-52 „ 

 55000 „ 

 001 fringe 



57-00 mm. 

 52-00 „ 

 2-50 „ 



7-50 „ 



440-00 „ 



1-5 X 10-5,, 



44-00 mm. 

 40-00 „ 

 2-00 „ 



4-00 „ 

 550-00 „ 

 3x10-6,, 



Value of least reading 



Ercolini states that each of the tubes operated on expe- 

 rienced at the moment of charge an elongation, which dis- 

 appeared with the discharge. But that these results were so 

 small and disturbed by oscillations occurring when the spark 

 passed that he could not obtain numerical results. Together 

 with this elongation of the tube were lateral displacements 

 shown by irregular movements of the bands, which could 

 never be avoided. These were probably due to irregularities 

 in the thickness of the glass and to lack of coincidence in the 

 axes of the three tubes. 



Just how the w riter could distinguish between the regular 

 and irregular displacement of the bands, it is difficult to see, 

 since they must have occurred simultaneously ; nor does he 

 take any notice of effect of the heating of the tubes. 



Dr. Sacerdote t considers this case to require the special 

 equation 



= -^1. 



Stt 



That is, the elongation is dependent only on the difference of 

 potential, the dielectric constant of the glass, the variation 

 of this constant with pressure ki, and the distance between 



* Ercolini, Nuovo Chnento, ser. 5, t. ii. p. 5 (1901). 

 t Ercolini, Journ. de Phys. ser. 4, t. i. p. 40. 



