GALVANISM AND TERRESTRIAL MAGNETISM.

Auckland Star, Volume II, Issue 552, 17 October 1871, Page 2

GALVANISM AND TERRESTRIAL MAGNETISM.

ON Monday at half-past seven o'clock, Mr. H. A. Severn delivered another lecture upon certain branches of physical science in tho Choral Hall, Symonds-street. Although the weather was anything but prepossessing, the audience was good, better, in fact, than the attendance <.n the former occasion. The subjects brought under review last night were—galvanism, terrestrial magnetism, magnetism and electromaguetism, the electric light, and the combustion of 'magnesium wire. Wlien the lecturer, took his p^ace on the platform he was warmly applauded, and commenced by stating that the way the attention of scientific men was first druwn to the attractive properties possessed by loadstone, was an accidental occurrence in Siberia. He explained the general properties of loadstone, and its prooerty of magnetising a Bteel bar when rubbed with it, bo that the property of polarity was thence imparted to the. steel bar. A bar so magnetised pointed to the magnetic nerth and south poles, and thus produced the needle of the mariner's compas*. He had no specimens of loaditcuie With him, and therefore oould not illustrate what he stated on that subject. Ho referred to frictional electricity, and showed the plate for a large electrical machine which ha expected to have finished in the course of a week, and stated that he expected to obtain sparks or lightning from it at least 18 inohes long. He rapidly enumerated various services whioh electricity performed for man, such as exploding submarine mines, torpedoes, electric telegraph, &o. He illustrated the effeofc produced by a very sma 1 galvanic battery, by connecting it with ft bell at a considerable distance from the platform, when the bell was at once set a-ringing. The term galvanism he did not approve of, and considered that as it was a chemical production, it should be called chemical electricity. The variation of the compass in different longitudes was clearly explained by reference to a large diagram at the end of the hall, showing that the magnetic pole did not coincide with the true poles of the earth, and thus the terrestrial and magnetic meridians did not agree with each other. On certain pavts of the earth's surface they vbry nearly coincided, and at those points tho magnetic north and tke true north made the nearest approach to each other, or, in other words, the variation of the compass was the least. Terrestrial magnetism was caused by the heat of the sun acting on the ecliptic ot the earth. The magnetic equator was at the part of the earth's Burface which was in the sun's path, and there the.magnetism was produced, which was distributed over tbo globe. After briefly referring to the spots on the sun, and stating that at times when they were most numerous magnetic storms were.most frequent, he corrected the commonly entertained idea that during a thunder-storm the lightning was attracted to the earth. There was no attraction of that kind. When the clouds became surcharged with electricity it sought the most direct way to the earth, and if two lightningconductors were erected, one of iron and one of copper, near to each other, the lightning would descend the copper road, not because it had any attraction for the electrical discharge, but because it offered the least resistance for it in its passage to the earth. If a lightning-conductor were too small a heavy charge of electricity would fuse it, but he believed that a strip of copper, two inches wide, would safely conduct to the earth the largest flash of lightening that would ever descend to the earth. He illustrated the power of eletromagnetism by coiling round a steel rod a ureat length of copper wire, and connecting it neotion with the battery was unbroken, the steel bar was a most powerful magnet, and he stated that while in Melbourne, he had lifted one and a half tons by sush simple means. The instant tho wire is disconnected with the battery the magnetic property of the steel ceased. It was that property that attempts had been made to utilize in the production of electro-magnetic engines. The power was ft wonderful one, and many years ago he had been on board a boat going at the n'te of eight miles an hour by these means, b <ilt by Napier, of Lancefield Works, on the Clyde. He exhibited the working of the electric telegraph, the decomposition of water, &c, all of which were very successful. He stated that when water wus so decomposed into its elements—hydrogen and dxygen—one pint of water produced ah effect tiu uui to one ton of coal; The electrio light whs exhibited on a small scale, and he Btated that a very complete apparatus of that kind had been ordered from England, and in less thun a twelvemonth he hoped to be able, by its means, to enable a person to read a lester at the North Shore in the darkest night. The fusion of a piece of magnesium wire was^ most successfully performed. At various' times during the leotur« the illustrations were warmly applauded.—Next Monday evening another lecture will be given.