XVIII. On repulsion resulting from Radiation.—Part II

Abstract

The present paper is in continuation of one which I had the honour of reading before the Royal Society, December 11th, 1873, and which was published in the Philosophical Transactions, vol. clxiv. part 2, page 501. In that paper I described various pieces of apparatus, chiefly in the form of delicate balances suspended in glass tubes, by means of which I was enabled to show attraction or repulsion when radiation acted on a mass at one end of the beam, according as the glass tube contained air at the normal pressure, or was perfectly exhausted. At an intermediate internal pressure the action of radiation appeared nil . Towards the end of the paper 1 said (70), “I have arranged apparatus for obtaining the movements of repulsion and attraction in a horizontal instead of a vertical plane. Instead of supporting the beams on needle-points, so that they could only move up and down, I suspend them by the centre to a long fibre of cocoon-silk in such a manner that the movements would be in a horizontal plane. With apparatus of this kind, using very varied materials for the index, enclosing them in tubes and bulbs of different sizes, and experimenting in air and gases of different densities up to Sprengel and chemical vacua, I have carried out a large series of experiments, and have obtained results which, whilst they entirely corroborate those already described, carry the investigation some steps further in other directions. I have introduced two important improvements into the Sprengel pump which enable me to work with more convenience and accuracy. Instead of trusting to the comparison between the barometric gauge and the barometer to give the internal rarefaction of my apparatus, I have joined a mercurial siphon-gauge to one arm of the pump. This is useful for measuring very high rarefactions in experiments where a difference of pressure equal to a tenth of a millimetre of mercury is important. By its side is an indicator for still higher rarefactions; it is simply a small tube having platinum wires sealed in, and intended to be attached to an induction-coil. This is more convenient than the plan formerly adopted (51), of having a separate vacuumtube forming an integral part of each apparatus. At exhaustions beyond the indications of the siphon-gauge I can still get valuable indications of the nearness to a perfect vacuum by the electrical resistance of this tube. I have frequently carried exhaustions to such a point that an induction-spark will prefer to strike its full distance in air rather than pass across the 1/4 inch separating the points of the wires in the vacuum-tube. A pump having these pieces of apparatus attached to it was exhibited in action by the writer before the Physical Society, June 20th, 1874.