a Dielectric in an Electrostatic Field. 359 



whence 



a? = iuD micron *' 



which corresponds to only 3j divisions of the micrometer eye- 

 pi ece f— a displacement which evidently could not be observed 

 with certainty, since on pp. 202, 203 of his paper Prof. More 

 mentions displacements of the zero of the micrometer which 

 exceed 6 divisions J. 



Discussion of the First Experiment. —In the second experi- 

 ment we had to deal with a thin condenser with adherent 

 armatures ; the dilatation being in this case given by the 

 formula § J 



7 =(^*i)~p (1) 



H denoting the intensity of the electric field in the glass 

 (H = V/e) . In the first experiment we have, on the contrary, 

 a condenser with armatures independent of the glass tube 

 since they are separated ^ from it by a liquid dielectric; the 

 dilatation is thus given by the formula || 



81 . Kh' 2 



T = ^^r' (2) 



h denoting the new intensity of the electric field in the 

 glass. But as : 



(1) The coefficient \ is considerably smaller than 



(2) The field h is much less intense than H, since the 



potential-differences were the same and the distance 

 between the armatures much greater. 

 The elongation 81 will thus be much smaller still than A/ 

 and consequently absolutely inappreciable. 



* For a spark-leno-th f 20 mm (V—~lWnn C! \ "' 3 

 is to say less than ^dlvisions^a^lifmefei^ * = 100 "-that 



ass -1 - to ae ^ th » d ^^^ e x^77 e 



itTnot't), p. 3l Cted fr ° m ' ^« tat! - ^& ( '' P ' 308) ' 



Mal.l9M ie Pr0 '° f ^ tUs f ° rmula ' see Sac3rfote,^.<fePVI'eb.- 



11 (See Sacerdote, loc. cit. p. 372 ) & i<? «f «,„ «wi<„. in to 

 a nearly equal to l'6x 10-12 c.G.s del 10 ~ 12c ' G - s - and 



