Page:Popular Science Monthly Volume 7.djvu/709

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THE MECHANICAL ACTION OF LIGHT.

ferently. Fig. 4 represents the best form: a b is a glass tube, to which is fused at right angles another narrower tube, c d; the vertical tube is slightly contracted at e, so as to prevent the solid stopper d—which just fits the bore of the tube—from falling down. The lower end of the stopper, d e, is drawn out to a point; and to this is cemented

PSM V07 D709 Light measuring apparatus.jpg
Fig. 4.

a fine glass thread, about 0.001 inch diameter, or less, according to the torsion required.[1]

At the lower end of the glass thread an aluminium stirrup and a concave glass mirror are cemented, the stirrup being so arranged that it will hold a beam, f g, having masses of any desired material at the extremities. At c in the horizontal tube is a plate-glass window cemented on to the tube. At b is also a piece of plate-glass cemented on. Exhaustion is effected through a branch-tube, h, projecting from the side of the upright tube. This is sealed by fusion to the spiral tube of the pump. The stopper d e and the glass plates c and b are well fastened with a cement of resin and bees'-wax.

The advantage of a glass-thread suspension is that the beam always comes back to its original position.

An instrument of this sort, perfectly exhausted and then sealed off, is shown at work in Fig. 5. It has pith-plates at the extremities of the torsion-beam. A ray of light from the lamp is thrown on to the central mirror, and thence reflected on to the graduated scale. The approach of a finger to either extremity of the beam causes the

  1. Some of the glass fibres used in these torsion-balances are so fine that when one end is held between the fingers the other portion floats about like a spider's thread, and frequently rises until it takes a vertical position.