6 Prof. L. T. More on Electrostriction. 



Glass rod with capillary bore, length 45 cms., diameter 

 5 mm. Potential from 14 to 80 kilo volts. No movement 

 whatever was observed except when a spark passed. 



Glass tubes 75 cms. long, wall-thickness from 0*08 to 

 0*12 cm. Potential in each case great enough to rupture the 

 tube. No movement so large as one-half a band-width was 

 observed, when the conditions were such as to avoid a direct 

 heating effect. (With tubes of the same length and 0*13 cm. 

 thick, Cantone records a displacement of 1*4 band for a spark- 

 length of 1 centimetre.) 



Glass tubes, silvered part of the length 73 cms., wall- 

 thickness 0*24 cm. Potential produced a spark 5 cms. long, 

 at which distance the tubes broke. The writer shows that, 

 compared with Cantone^s results, the expansion should have 

 been far within the limits of possible readings with the 

 instrument. No effects were observed other than those plainly 

 due to temperature changes. Not the least motion of the 

 bands could be detected when the potential was instantaneously 

 raised by connexion being made with a set of highly charged 

 jars. 



Hard rubber tubes, length 100 cms., wall- thickness 2*5 mm. 

 Potentials increased to 4 cms. spark, when tubes broke. Ac- 

 cording to Korteweg and Julius, the expansion of rubber is 

 of the order 10 ~^, instead of 10 ~^^, the coefficient of glass. 

 When charged for one minute, there was a slow motion of 

 18 bands across the field. About ten minutes were required 

 for the complete return. This motion was shown to be due 

 to heating. A long piece of fine copper wire was wound 

 spirally about a similar tube. The wire was inserted in a 

 Wheatstone-bridge and the resistances balanced. The tube 

 was then charged with a potential to give a 2-cm. spark, the 

 displacement of the fringes noted, and the tube discharged. 

 On again measuring the resistance of the wire it was found 

 to be greater, and the increase was measured. Calculation 

 of the rise in temperature of the wire proved that the heating 

 of the tube was of the same order as the elongation of the 

 tube, if it were due to heating alone. 



Dr. Shearer concludes from his experiments that the ob- 

 served changes in length may be readily accounted for 

 without reference to other causes than heat and slight dis- 

 tortions due to an unsymmetrical field of force ; and that the 

 production of changes of form and dimensions in matter by 

 purely electrostatic charge is extremely improbable. 



For the theoretical development of this subject the reader 

 may refer to the memoirs of Sacerdote and Shearer, who have 

 given complete bibliographies. 



