Open main menu

Page:Popular Science Monthly Volume 21.djvu/266

This page has been proofread, but needs to be validated.
256
THE POPULAR SCIENCE MONTHLY.

supposed by Professor Bjerknes and Professor Forbes to be due, not to the action of one body on the other, but to the mutual action of one body and the water in contact with it, the water being regarded as the analogue of Faraday's medium. "Viewed in this light," says Professor Forbes, "his first experiment is equivalent to saying that, if a vibrating or oscillating body have its motions in the same direction as the water, the body moves away from the center of disturbance; but, if in the opposite direction, toward it. This idea gives us the analogy of diaand para-magnetism. If, in the neighborhood of the vibrating drum, we have a cork ball, retained under the water by a thread, the oscillations of the cork are greater than those of the water in contact with it, owing to its small mass, and are, consequently, relatively in the same direction. Accordingly, we have repulsion, corresponding to diamagnetism. If, on the other hand, we hang in the water a ball which is heavier than water, its oscillations are not so great as those of the water in its vicinity, owing to its mass, and consequently the oscillations of the ball relatively to the water are in the opposite direction to those of the water itself, and there is attraction corresponding to para-magnetism. A rod of cork and another of metal are suspended horizontally by threads in the trough, and a vibrating drum is brought near them: the cork rod sets itself equatorially, and the metal rod axially."

Fig. 2.
PSM V21 D266 Simple vibrators producing electromagnetism.jpg

From these and other experiments Professor Bjerknes has concluded that the motion of a vibrating agent in the water produces a real magnetic field, with its lines of forces presenting, but always in an inverse sense, identical phenomena of diamagnetism and para--