INTENSE MAGNETIC FIELDS.

Otago Daily Times, Issue 20315, 25 January 1928, Page 4

INTENSE MAGNETIC FIELDS.

HIGH VOLTAGES. SIR E. RUTHERFORD ON RECENT RESEARCH. (From Our Own Correspondent.) LONDON, December 14. lu his presidential address at the anniversary meeting of theTßoyal Society, Sir Ernest Rutherlord referred to the results of .nvestigations which have been carried out m recent years to produce intense magnetic fields and high voltages for general scientific purposes. By means of modern electrostatic machines, he said, it was not difficult to produce weak direct currents at potentials from 200,000 to 300,000 volts, while a large well-insulated induction coil could give momentary \ 7 °h tages of a similar magnitude. The wide ueo of X-rays for diagnostic and therapeutic purposes had led to a marked improvement in apparatus for exciting intense X-rays. The requirement of very penetrating X-rays for deep therapy in our hospitals had led to the construction of comparatively light transformers, which would supply the requisite small currents at voltages between 300,000 and 500,000. One of the simplest ways of producing very high voltages was by the Tesla transformer; with that it was not difficult to produce voltages .of the order of 1,000,000 volte, and he understood as much as 5.000. volts had been obtained in the Carnegie ’lnstitute at Washington. The rapid frequency of the oscillations and the comparatively small energy given to the secondary of a Tesla coul had, however, restricted its use for general technical purposes as a source of high voltages, although it was now finding an application for the testing of insulating materials. A CASCADE METHOD. Tlie increase of the operating voltage of electrical transmission lines had led to the need of very high voltages to test the insulating properties of those lines and their transformers and the effect of electric surges in them. During the last few years a number of high voltage plants had been installed for testing purposes in various countries, which gave from 1.000. to 2,000,000 volts. Those voltages might be obtained either by a very large well-insulated power transformer or, more generally, by a cascade method employing several transformers. There appeared to be no obvious limit to the voltages obtainable by the cascade arrangement, except that of expense and the size of tha building required. He was informed that the Genera! Electric Company, of Schenectady, had a working plant giving a maximum of 2,800,000 volte, and hoped soon to have one to give 6,000,000 volts. HIGH VELOCITY ELECTRONS. While no doubt the development of such high voltages served a useful technical purpose, from the purely scientific point of view interest was mainly centred on the application of those high potentials to vacuum tubes in order to obtain a copious supply of high-speed electrons and high-speed atoms. A vigorous attack on that side of the question had been recently undertaken by Dr Coolidge who, taking advantage of the great improvements in vacuum technique and the ease of supply of electrons from a glowing filament. had constructed an electron tube, which would stand 300,000 volts, the rays passing into the air through a thin plate of chrome-nickel-iron alloy about O.OOOSin thick. It had not so far been found practicable to apply much more than 300,000 volts to a single tube, on account of the danger of a flash over. For the application of still higher voltages a number of tubes were arranged in series and communicating with one another, the fall of potential in each being about 300,000 volts. So far experiments had been made with three tubes in scries and 900,000 volts, which gave a supply of electrons corresponding to one or two milliamperes through the thin window in the last tube. That gave an intense beam of high-velocity electrons, which spread out into a bemisphere extending to a distance of about two metres from the window. Marked luminous effects wore produced in the air iteself and in phosphorescent bodies placed in the path of the rays. INTENSE FIELDS. Electro-magnets were ordinarily employed for the production of intense magnetic fields, and the magnetic fields obtainable were in th 6 main limited by the magnetic saturation of the iron. In order to provide fields of the order of half a million gauss, the use of the elec-tro-magnet must be abandoned. Some years aga, Dr Kapitza suggested that intense momentary magnetic fields could be obtained by sending a very strong current through a coil for such a short interval that the heating effect in the coil was restricted to a permissible value. The main difficulty had been to construct a coil strong enough to withstand the enormous disrupting forces which arose when a large current was passed through it. By special attention to the design, a coil had been constructed which gave a field of 320,000 gauss over a volume of about 3c.c. without any signs of fracture. Measurements had been regularly carried out in fields of that magnitude. It was anticipated that the present design of coil would give about 500,000 gauss before it burst, and that still higher fields could be obtained in coils specially constructed for the purpose. The investigations, which had been carried out in the Cavendish Laboratory, Cambridge, had been made possible by the generous support of the Department of Scientific and Industrial Reeaarch. The application of those new methods of producing intense fields opened up a wide region of research, where all magnetic properties could be examined in fields 10 to 20 times stronger than those hitherto available.