262 Prof. S. P. Thompson on the Electric 



desired than those that are presented here ? There is, first, 

 the directional transmission of electric force from limit to limit 

 of the field ; there is, secondly, the special dioptric action of 

 the force-transmitting medium, an action precisely similar to 

 that of glass directionally strained hy tensions or pressures; 

 and there is, finally, the directional rupture of the dielectric 

 when the intensity of the transmitted force reaches a critical 

 value characteristic of the medium. 



I insist upon the essentially directional character of electric 

 strain, in opposition to a theory of electro-optic action which 

 has been advanced by Professor Quincke*. 



Electro-optic double refraction may be attributed to a quasi- 

 crystalline and uniaxal structure, which is maintained by 

 electric stress in the medium. I advanced this view of the 

 phenomena in my first paper on the subject ; and I still believe 

 that it contains a germ of the truth. I think indeed that, to 

 any one engaged with the facts of Electro-Optics, there is 

 no theoretical conception that can present itself more naturally 

 than that of an electrically induced structure, a regular 

 arrangment of polarized molecules, roughly illustrated by the 

 common " magnetic curves." 



XXXII. On the Electric Resistance of Caroon under Pressure. 

 By Professor Silvanus P. Thompson, B.A., D.Sc.\ 



§ 1. TT has often' been stated that the electric resistance of 

 -I- carbon decreases when subjected (1) to an increase 

 of temperature, (2) to a mechanical compression. The first 

 of these statements has been verified by so many authorities 

 that there can hardly be any question of its correctness. The 

 second I believe to be wholly misleading ; for some careful 

 experiments that I have lately made lead to the conclusion 

 that the effect of mechanical compression upon the electric 

 resistance of dense carbon is almost, if not quite, nil. and 

 that what has been mistaken for an increase in electric con- 

 ductivity is in reality merely better contact at the points of 

 junction with the circuit. 



§ 2. A preliminary experiment to test whether the alleged 

 decrease of resistance by pressure was due to a true increase 

 in specific conductivity or to better end-contact, was made in 

 the following manner : — A thin cylindrical rod of Carre's 

 dense artificial carbon (such as is used in electric lamps) was 

 taken, its length being 72 centimetres. At a point about one 



* Phil. Mag. for July 1880, p. 37. 



f Communicated by the Physical Society, having been read at the 

 Meeting on February 25, 1882. 



