Prof. L. T. More on Electrostriction. 15 



ir< incorrect, as it should be 



^vhere ijl = modulus of elasticity = 7 x 10^^ 



K = dielectric constant = 7 



V = potential-difference = 110 C.G.s. 



a — Poisson's ratio = 0*25 



/ = length = 50 cms. 



d = thickness = 0*2 cm. 



or 8Z = 1"5 X 10~^ cm. = 3 divisions of micrometer. 



This effect is then, at least, one-fifth the elongation when a 

 tube is charged for the first time, and about one-half the elon- 

 gation if it has been previously charged to a higher potential. 



There can be no doubt that the charges on the armatures are 

 sufficient to heat the glass an appreciable amount, for as soon 

 as the tube has been discharged, the image begins to move 

 continuously in the direction to show an elongation. The 

 values in Table VI. give the magnitude of this effect. With 

 a spark-gap of 16 millimetres, an initial deflexion of 15 

 divisions was obtained, the same as for a 12 millimetre spark. 

 The tube was then discharged and allowed to stand five 

 minutes, during which time the image moved 92 divisions 

 (from 35 to 193). A second charge gave a deflexion of 26 

 divisions and a discharge for one minute 44 divisions. The 

 change of seven divisions during the ten minutes following 

 has no significance, as it was necessary to readjust the 

 apparatus. But the effect of heat in expanding the tube is 

 even more apparent from the values obtained when the tube 

 was charged. The first reading, with an unheated tube, is 15 

 divisions. If it is then allowed to cool for ten minutes 

 before charging again, the elongation is 18, and for five 

 minutes 26 ; while if the previous cooling is only for one 

 minute the expansion is 41. It seems evident that the increase 

 of length, due to heat evolved during the charging, is of an 

 order equal to the observed eftect. 



It is easy to calculate the rise in temperature which 

 would expand the tube the entire amount, assuming 15 

 divisions or 9 X 10~^ centimetre for a 12 millimetre spark. 

 The formula is 



where 8L = observed expansion = 9 x 10~^ cms. 

 \ = coeff. of expansion = 9xl0~® 

 L = length of tube = 60 cms., 



then ht = 0°-017 C. 



