124 STATIC ELECTRICITY 



By this method Boltzmann obtained the following results . 



Ebonite K = 3'15 



Paraffin J2-:W 



Sulphur (non-crystalline) 3*84 



Resin 2'55 



The residual effect in these cases was practically negligible, for 

 the effect was the same whether the contacts only lasted for a fraction 

 of a second or whether the operation lasted from one to two 

 minutes. With glass, gutta-percha, and oilier less perfect insulators, 

 however, the residual effects were so great that the method was 

 inapplicable. 



The square of the refractive index for paraffin for an infinite 

 wave-length is calculated to be 2 022. That for sulphur for the 

 D line is a little over 4. That for ebonite * : ; d rays 



is about 2'76. That for resin is given bv Holt /maun as 2*38. 

 The dielectric- constant and the square of the refract i\e in- 

 then, are not very different in these c.t 



Boltzmann's investigations with crystalline sulphur, f 



We have seen that if a conducting sphere radius a \^ placid in a 

 uniform field E the external field is charged as if there were an 

 electric doublet at the centre of imimci ^impose t 



M is charged with Q, and that n conducting sphere N, radnis 

 placed at a distance d from its centre, and is so small that tin- field 

 round N due to M may be regarded as uniform ami i<*ity 



E = i. The distribution on N is externally equi\ alent to a doublet 



^. The pull by it on M will then-fore- lie *Q* if we neglect t he 



u> C* 



effect of N in disturbing the charge on M. 1 M on 



N will be equal to this. 



Now replace N by a dielectric sphere of the same siie with 



constant K. The dielectric sphere is equivalent to a doublet 

 j _ I 



y , a f that representing the conducting sphere, and the pull on 



K 1 



the dielectric due to M will only be .. of that on the , 



ducting sphere. 



Boltzmann arranged a small conducting sphere suspended bv 

 silk threads from one end of a torsion arm, and mcaMin d the 

 deflection when an elect riti< ^ in its neighbourhood. 



The "ondiu-ting sphere was then repla. <d In an equal cr\stalline 

 sulphur Sphere, and the pull was again n. when the three 



axel of the crystal were in succession din* uds the atti 



ing sphere. The ratio of the pulls on the conducting sphere and 



* Ayrton ami Prrrv. //'/. .!/.; 



t Wiener SittMiytbcncktc, Ixx. part ii. p. 342 ; Gordon, he. cit. p. 100. 



