For these mechanical effects the exhaustion need not be so high as when phosphorescence is produced. The best pressure for this electrical radiometer is a little beyond that at which the dark space round the negative pole extends to the sides of the glass bulb. When the pressure is only a few millimetres of mercury, on passing the induction current a halo of velvety violet light forms on the metallic side of the vanes, the mica side remaining dark. As the pressure diminishes, a dark space is seen to separate the violet halo from the metal. At a pressure of half a millimetre this dark space extends to the glass, and rotation commences. On continuing the exhaustion the dark space further widens out and appears to flatten itself against the glass, when the rotation becomes very rapid.
Here is another piece of apparatus (Fig. 13) which illustrates the mechanical force of the radiant matter from the negative pole. A stem (a) carries a needle-point in which revolves a light mica fly (b b). The fly consists of four square vanes of thin, clear mica, supported on light aluminium arms, and in the center is a small glass cap, which rests on the needle-point. The vanes are inclined at an angle of 45° to the horizontal plane. Below the fly is a ring of fine platinum wire (c c), the ends of which pass through the glass at d d. An aluminium terminal (e) is sealed in at the top of the tube, and the whole is exhausted to a very high point.
By means of the electric lantern I project an image of the vanes on the screen. Wires from the induction-coil are attached, so that the platinum ring is made the negative pole, the aluminium wire (e) being positive. Instantly, owing to the projection of radiant matter from the platinum ring, the vanes rotate with extreme velocity. Thus far the apparatus has shown nothing more than the previous experiments have prepared us to expect; but observe what now happens. I disconnect the induction-coil altogether, and connect the two ends of the platinum wire with a small galvanic battery: this makes the ring c c red-hot, and under this influence you see that the vanes spin as fast as they did when the induction-coil was at work.
Here, then, is another most important fact. Radiant matter in these high vacua is not only excited by the negative pole of an induction-coil, but a hot wire will set it in motion with force sufficient to drive round the sloping vanes.
Radiant Matter is deflected by a Magnet.—I now pass to another property of radiant matter. This long glass tube (Fig. 14) is very highly exhausted; it has a negative pole at one end (a) and a long phosphorescent screen (b, c) down the center of the tube. In front of the negative pole is a plate of mica (b, d) with a hole (e) in it, and the result is, when I turn on the current, a line of phosphorescent light (e, f) is projected along the whole length of the tube. I now place beneath the tube a powerful horseshoe magnet: observe how the line of light (e, g) becomes curved under the magnetic influ-
