Action of Tesla Coil on Radiometer

The following phenomena, observed while experimenting with a small Tesla coil, will, I believe, interest some of your readers. Not having access to the necessary literature, I am not in a position to find out whether they are new or already known.

The knobs of the Tesla coil were placed in contact with, or just close to, the bulb of a Crookes's radiometer, and the coil set at work. When the brush discharge fell upon the bulb, the blackened surfaces of the vanes first retreated, as they do under the influence of radiant heat, but soon the direction of rotation changed, and the blackened surfaces moved forward, the motion continuing as long as the brush discharge fell upon the bulb.

At the same time, inside the bulb, were seen diverging from the glass sides close to the knobs two cones of pale blue light, which, falling on the opposite sides of the bulb, caused a yellowish-green fluorescence. On the fluorescent parts the shadows of the rotating cones were clearly visible, the shadow on one side being always more intense than on the other side. When the direction of the current in the charging Ruhmkorff was reversed, the shadows exchanged places, but no change in the direction of rotation of the vanes was noticed.

On examining the fluorescent parts with a screen of potassium platinocyanide, the same effects were noticed as with the X-ray tubes. 

Similar effects were obtained on repeating the experiments with two incandescent lamps in the laboratory. The larger of these, an old Swan lamp, fluoresced green, and the smaller new one, supplied with the Tesla coil by the manufacturer, fluoresced blue. But in both cases, though somewhat feeble, the same X-ray effects were observed.

To study further the cause of the motion of the vanes of the radiometer, the experiment was repeated with a Crookes's tube containing a freely suspended wheel with transparent mica waves. In this case it was found possible to alter the direction of rotation of the wheel by adjusting the positions of the knobs of the Tesla coil relatively to the sides of the tube and the wheel inside it.

P. L. NARASU.
Christian College, Madras, June 18.