Page:Collected Physical Papers.djvu/39

 III

ON DOUBLE REFRACTION OF THE ELECTRIC RAY BY A STRAINED DIELECTRIC

The following investigations were undertaken to find whether a solid dielectric becomes double-refracting to the electric ray when it is subjected to a molecular stress by unequal expansion in two directions, or by mechanical compression.

The Electric Polarisation Apparatus, previously described, was used for these experiments. The Analyser and the Polariser were crossed. The strained substance was then introduced between the crossed Analyser and Polariser.

Effect due to unequal expansion

I cast a rectangular piece of paraffin, and chilled its surface unequally by a freezing mixture. On suitably interposing this between the Polariser and the Analyser, the galvanometer spot was at once deflected, proving the double-retracting nature of the strained dielectric. The piece was cast six months ago. It retains its unannealed property even now.

I took two zinc tubes, one cylindrical and the other rectangular, and made castings of melted pitch, which set on cooling. In the cylindrical tube the strain was the same in all directions at right angles to the axis, and this cylindrical mass of pitch did not exhibit any double-refraction. But in the tube with the square section, unequal strain was evident in rectangular directions, perpendicular to the axis; two of the opposite (19)