The Oersted Considered as a New International Magnetic UnitSource: The Scientific Monthly, Vol. 32, No. 4 (Apr., 1931), pp. 378-380Published by: American Association for the Advancement of ScienceStable URL: http://www.jstor.org/stable/14876 .

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8 78 THE SCIENTIFIC AIONTHLY

'... Svar liVl )~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~... . ...

pW // :/

.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~J . .. ..

I

=~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.. . !.....! __.

THE STATUE OF OERSTED IN COPENHAGEN

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THE PROGRESS OF SCIENCE 379

THE OERSTED CONSIDERED AS A NEW INTERNATIONAL MAGNETIC UNIT

TH1ERE are eight electrical units, whose names by international agreement are used all over the world in electrical sci- ence and industry. Two or three of these are in such common use that their names are familiar to a large section of the general public. The two most gen- erally known are, perhaps, the watt and the volt. rhe watt, which is the unit of power, or rate of doing work of any kind (mechanical, thermal, electrical, chemical, etc.) is named after James Watt, the Scottish scientist, inventor and engineer. James Watt revolution- ized the design and construction of the steam engine, and in order to measure the power of his engines, he determined the average working rate of certain brewery horses engaged in pumping water steadily from a known depth. To a certain rate of lifting weight so de- rived, he gave the name horse power. It probably never occurred to him that his

own name would be subsequently ap- plied to an international unit of power.

In a similar manner, the volt, which is the unit of electric tension or electro- motive force, is named after the Italian physicist Alessandro Volta, the discov- erer and first inventor of the voltaic cell.

The complete series of international electrical units, as adopted up to date, is given in the accompanying table. Numbers 9 and 10 in the table are mag- netic units adopted at the International Electrical Congress of Paris in 1900. As is indicated in column III, there was a certain ambiguity about the nature of the gauss at the time of its adoption. The proposers of the gauss intended it to be the unit of magnetic flux density B; but, by a misunderstanding, it appeared to have been adopted as the unit of mag- netizing force H. There was subse- quently a considerable amount of con-

TA:BLE OF ELECTRIC AND MAGNETIC UNITS

Adopted Numiber Unit Symbol For Named Lived Country At name electric after Year At

1 Volt E Tension A. Volta 1745-1827 Italy 1881 Paris 2 Ohm R Resistance G. S. Ohm 1778-1854 Germany 1881 Paris 3 Ampere I Current A. M. Ampere 1775-1836 France 1881 Paris 4 Coulomiib Q Quantity C. A. Coulomb 1736-1806 France 1881 Paris 5 Farad C Capacitance M. Faraday 1791-1867 England 1881 Paris 6 Joule JIV Work J. P. Joule 1818-1889 England 1889 Paris 7 Watt P Power J. Watt 1736-1819 Scotland 1889 Paris 8 Ilenii3y L i-nductanee J. IIenry 1799-1878 Amnerica 1893 Chiicago

For magnetic

9 Maxwell (P Flux J. C. Maxwell 1831-1879 Eiigland 1900 Paris 10 Gauss HB {Force K. F. Gauss 1777-1855 Germany 1900 Paris B P lux density 11 Gilbert F Magneto-

motive force W. Gilbert 1540-1603 E.ngland 1930 Oslo 12 Oersted H Force H. C. Oersted 1777-1851 Denmark 1930 Oslo

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380 THE SClENTIFIC MONTHLY

fusion in techniical literature, some writers using the gauss for the uniit of H, others for the unit of B, and still others for both. The matter was further complicated by the fact that the quan- tity H has been used in two different senses; namely, (1) the magnetizing force due to the excitingv current-turns linked with a magnetic circuit, and (2) thte intensity of the magnetic field pro- duced by the exciting current when the magynetic circuit is a vacuum or contains no miiagnetic material. Differences of opinion and of usage became so numer- ous and wide-spread that the matter was referred to the meeting of the In- ternational Electrotechnical Commission (1. E. C.) in Scandinavia last summer (June-July, 1930). At that miieeting. the I. E. C. decided that, for electro- technical purposes, magnetizing force H should be regarded as essentially differ- ent from field intensity B., or flux den- sity B. To the latter the unit name of gaiss should be restricted; while the nam-e oersted was adopted for the unit of magynetizing force H, by way of dis- tinction.

IHans Christian Oersted (in Latin, Johannis Christianus {)rsted was, in- 1820, professor of plhysics in the Univer- sity of Copenhagen, Denmark. In tlhe springr of that year, he made laboratory experiments in search of some connec- tion between magrnetism and electricity. U,p to that date, those two sciences were regarded as unconnected and indepen- dent. The science of electricity was well recognized, and also the science of magnetism-essentially pertaining to permanent magnets; but there was no suchl science as clectromagnetism. He discovered that when a wire, carrying a steady electric current, was brought into the neighborhood of a horizontally

suspended miiagnetic needle, such as the needle of a mariner 's compass, the needle was deflected from its normal north-south position, and remained so deflected as long as the current flowed in the wire, or as long as the active wire remainied in the needle's vicinity. This remarkable experiment, repeated in a number of different ways, was described by Oersted in a circular letter, prinited in Latin, at that timiie an interniational language in considerable vogue, and ad- dressed to a niumber of universities and learned societies throughout the world, bearing the date of July 21, 1820.

Ever since 1820, electricity and magr- netism have been regarded as inldis- solubly connected. The union led shortly afterwards, thirough the work of other scientists, to further discoveries in electromagnetism wlich have pro- foundly affected the coniduct of civilized life.

The accompanying illustration is from a photograplh of a statue of Oersted, which has been erecte(I to the imiemory of this distinguished Danislh discoverer in a park at Copen-haaeni, known as tlhe Oersted Park. It represents the dis- coverer standing beside a pedestal bear- ing a freely supported magnietic needle which he is showing to be deflected by the influence of a current-carrying loop of wire leadiing to a voltaic battery at his feet. The front inscription reads "Hans Christian Oersted" and the in- scriptioni at the back, as translated from the D)anish, reads "Born 14th August, 1777, Died 9th Marel. 1851." On the occasion of its visit to Copenlhagen, Junie 27, 1930, the visitinga I. E. C. officers and delegates formally placed a suit- ably inscribed commemoration wreath in front of this statue.

ARTHIUR E. KENNELLY

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