A course of lectures on electricity delivered before the Society of Arts

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1891 Excerpt: ... proves rotation of the ether or of the molecules in the interior of magnetised bodies. We have now seen many connections between magnetic phenomena and purely mechanical motions; but, even at the risk of tiring you with the subject, I must draw your attention to another class of phenomena attendant upon the magnetisation of iron which was first discovered by Joule. He proved that when an iron bar is magnetised Fig. 10.--Bidwell'g apparatus. C is a coil of wire, It an iron rod working the lever, L, which has a fulcrum F The lengthening or shortening of the rod E is shoTM by the movements of a spot of light reflected from the mirror, M, on a pivot, P. its length is increased. It has been supposed by some that the cause is to be found in a greater length of the molecule of iron along its magnetic axis, so that when all the molecules are twisted so that their lengths all point one way the bar is lengthened. This view is rather weakened by the subsequent discovery of Mr. Shelford Bidwell, who has confirmed Joule's discovery so far as moderate magnetisation is concerned. But he finds that when the magnetisation is increased the bar ceases to expand, diminishes to its natural length, and with very powerful magnetising forces actually contracts to a considerable extent. As to the causes of these phenomena we are in darkness. But I feel confident that they are bound to tell us a great deal about the true physical meaning of magnetism. I consider myself fortunate in being able to show you with Mr. Bidwell's assistance the results which he has obtained. The apparatus used by him consists of a piece of iron rod fixed at the lower end, but pressing upwards on a lever. The long arm of that lever acts upon a mirror resting on a pivot, so that if the iron rod lengthens it...