12 Prof. L. T. More on Electrostriction, 



Employing the ordinary formula for Young^s modulus, we 

 have from the data, 



3. 4. 



Length of tube 65 cms. 65 cms. 



Area of cross-section 2*6 cms. 2*9 cms. 



Change of length per kg 60'0 div. 50*0 div. 



Value of a division 6x10"*^ mm. 6xlO~'^mm. 



Yomig's modulus (/t) Q'^x IQii 7-3 x 10^^ 



These results agree so closely with each other and with 

 the accepted value for this modulus for glass that there 

 should be no hesitation in accepting the readings of the 

 apparatus. 



Passing now to the experiments made when the condenser 

 was electrically charged, it is more convenient to give the 

 results in a series of tables without comment, and to discuss 

 them later. They may be di^dded into two classes, those with 

 adherent armatures and those when the armatures are 

 separated from the dielectric by a non-conducting fluid. 



Adherent Armatures, 



Tube 5 was cemented in place and covered on the outer 

 surface with tinfoil for a distance of 50 centimetres. The 

 space between this tube and the grounded brass tube was filled 

 with a concentrated solution of sodium carbonate in water, 

 which served to ground the inner surface of the glass tube. 

 A second glass tube, larger in diameter and without a metal 

 coating, was cemented coaxially with the others, and the sj^ace 

 between the glass tubes, about a millimetre, was filled with 

 pure lard oil. The oil and outer tube merely prevented 

 the charge from leaking off the tinfoil. A fine wire, con- 

 nected \N-ith the generator and spark-micrometer, was attached 

 to the foil ; first passing through a hole bored in the outer 

 tube, half-way up. The hole was then closed with a rubber 

 stopper. 



Observations are recorded only for days when the room 

 temperature was so constant that the image remained steady. 

 The drum of the micrometer contained 250 divisions, and a 

 change from a greater to a less number denotes an increase 

 in the length of the tube. Both tilting-mirrors were used, 

 but as the readings agreed they are all reduced to the 

 dimensions of the one with feet 6 millimetres apart. Observa- 

 tions are recorded in divisions of the micrometer, one division 

 being equivalent to 6 x 10~^ millimetres. 



From the final reading 243 with a 15 mm. spark, the 

 readings were made as rapidly as possible, shown by the final 

 reading of one being the same as the initial of the next. 



