| 释义 |
▪ I. ‖ ohm1|oːm|
[Ger., = aam.]
An obsolete German liquid measure equivalent to from 30 to 36 gallons according to the locality.
1851Longfellow Gold. Leg. iv. Convent Hirschau, It comes from Bacharach on the Rhine..And costs some hundred florins the ohm [rime Rome].
▪ II. ohm2 Electr.|əʊm|
[From the name of the German physicist Georg Simon Ohm (1787–1854), who determined mathematically the law of the flow of electricity (Ohm's law).]
1. The unit of electrical resistance: see quots. Now incorporated in the International System of Units, and defined as the resistance that exists between two points when a potential difference between them of one volt produces a current of one ampere.
The word ohma was orig. proposed for the practical unit of ‘tension’ (voltage), along with volt for the unit of resistance (see quot. 1861, which also appears in Electrician (1861) 9 Nov. 4 and is from the paper read at the Sept. 1861 meeting of the British Association). The origin of the change in application of the word has not been traced in print.
1861Clark & Bright in L. Clark Exper. Investigation Laws Propagation Electr. Current 49 Let us derive terms from the names of some of our most eminent philosophers... We shall then have the following table: A.—Tension. 1 Daniell's Element = 1 Ohma, or unit of tension... B.—Quantity. 1 Ohma, by 1 metre square at 1 milliometre [sic] distance = 1 Farad, or unit of quantity... D.—Resistance. 1 Farad per second = 1 Volt, or unit of resistance. Ibid., The ohma, or unit of tension, is practically a very convenient one for all battery purposes. 1865Proc. R. Soc. XIV. 159 It is proposed that the new standard [of resistance] shall not be..described as so many metre/seconds, but that it shall receive a distinctive name, such as the B.A. unit, or, as Mr. Latimer Clark suggests, the ‘Ohmad’. 1867W. H. Preece in Phil. Mag. XXXIII. 397, I beg to suggest..to those physicists and electricians who have adopted the British-Association unit for electrical measurements, that in place of expressing this unit, as is variously done at present, by B.A. unit, Ohmad, Ohm, 107 metre/second, &c., it would be very convenient to adopt some universal symbol analogous to that used for degrees (°). The Greek letter ω appears to me very convenient. Ibid., The conductivity of the Atlantic cable would be given by 7524ω, and its insulation by 2349Ω per knot, which may still be read Ohm and Megohm. 1867R. S. Culley Handbk. Pract. Telegr. (ed. 2) ii. 30 The unit [of resistance] adopted by the electricians of this country is that determined by a Committee of the British Association, and is called the ‘ohm’.., and sometimes the ‘B.A. unit’. 1870F. L. Pope Electr. Tel. iii. (1872) 25 The Ohm..is equivalent to about 1/16 of a mile of galvanized No. 9 iron wire. 1876Preece & Sivewright Telegraphy 5 It is convenient..to use a symbol to represent the ohm as we use ° to represent degrees, and ′ minutes. The symbol used by us is ω, the Greek omega. Thus we say that the resistance of a wire between London and Birmingham is 1500ω. 1889Nature 14 Feb. 368/2 It was in this country that..the term ‘ohmad’ or ‘ohm’, suggested by Sir Charles Bright and Mr. Latimer Clark at the meeting of the British Association in Manchester in 1861, first came into use as the name of a decimal multiple of the absolute unit of resistance convenient for practical purposes. At the Congress of Electricians in Paris in 1881, the Ohm was unanimously adopted as an international standard. 1892Gloss. Electr. Terms in Lightning 3 Mar. Suppl., The Ohm is the resistance of a column of mercury of a constant section of one square millimetre and of a length of 106·3 centimetres, at the temperature of melting ice. 1943F. E. Terman Radio Engineers' Handbk. ii. 40 Wire-wound variable resistors of these types are available up to about 100,000 ohms, and are capable of dissipating up to about 15 watts. 1973D. Aldous in Pye Bk. of Audio vii. 75/2 Most transistor amplifiers offer a wide range of output impedances, usually from 4 to 16 ohms.
2. attrib. and Comb., as ohm-centimetre, the unit of electrical resistivity in the C.G.S. system, being the resistivity of a substance of which a centimetre cube has a resistance of one ohm; ohm-metre, the corresponding unit in the International System of Units, equal to 100 ohm-centimetres.
1920W. T. Maccall Continuous Current Electr. Engin. (ed. 2) iii. 18 The resistance of a conductor 1 cm. long and 1 sq. cm. cross-section..is called the specific resistance per centimetre cube... A better name is ohm-centimetre (or microhm-cm.). 1957H. Cotton Electr. Technol. (ed. 7) iii. 67 It is..logical to express ρ as so many ohms-centimetre..or ohm-metre [sic]... For example, for copper at normal room temperature ρ = 1·72 × 10-6 Ω-cm = 1·72 × 10-8 Ω-m. 1974Encycl. Brit. Micropædia VIII. 524/1 If lengths are measured in centimetres, resistivity may be expressed in units of ohm-centimetre. Ibid., The resistivity of an exceedingly good electrical conductor, such as hard-drawn copper, at 20° C (68° F) is 1·77 × 10-8 ohm-metre, 1·77 × 10-6 ohm-centimetre, or 10·7 ohm-circular mils per foot.
Hence ˈohmad = ohm; ohm-ˈammeter, an instrument for measuring electrical current and resistance, a combination of an ammeter and an ohmmeter; ˈohmmeter, an instrument for measuring electrical resistance in ohms.
1866R. M. Ferguson Electr. 151 This is called the B.A. Unit of resistance 1864, or an Ohmad. 1891‘Electrician’ Primer No. 12. 8 Ohmmeters indicate the ratio of the pressure between the ends of a conductor to the current passing through that conductor. |