Report of the Forty-Third Meeting of the British Association for the Advancement of Science;
Held at Bradford in September 1873.
London: John Murray, 1874.
The Committee consisting of Sir W. Thomson, Professor G. C. Foster, Professor T. C. Maxwell, Mr. G. J. Stoney, Professor Fleeming Jenkin, Dr. Siemens, Mr. F. J. Bramwell, and Professor Everett (Reporter).
We consider that the most urgent portion of the task intrusted to us is that which concerns the selection and nomenclature of units of force and energy; and under this head we are prepared to offer a definite recommendation.
A more extensive and difficult part of our duty is the selection and nomenclature of electrical and magnetic units. Under this head we are prepared with a definite recommendation as regards selection, but with only an interim recommendation as regards nomenclature.
Up to the present time it has been necessary for every person who wishes to specify a magnitude in what is called “absolute” measure, to mention the three fundamental units of mass, length, and time which he has chosen as the basis of his system. This necessity will be obviated if one definite selection of three fundamental units be made once for all, and accepted by the general consent of scientific men. We are strongly of opinion that such a selection ought at once to be made, and to be so made that there will be no subsequent necessity for amending it.
We think that, in the selection of each kind of derived unit, all arbitrary multiplications and divisions by powers of ten, or other factors, must be rigorously avoided, and the whole system of fundamental units of force, work, electrostatic, and electromagnetic elements must be fixed at one common level—that level, namely, which is determined by direct derivation from the three fundamental units once for all selected.
The carrying out of this resolution involves the adoption of some units which are excessively large or excessively small in comparison with the magnitudes which occur in practice; but a remedy for this inconvenience is provided by a method of denoting decimal multiples and submultiples, which has already been extensively adopted, and which we desire to recommend for general use.
On the initial question of the particular units of mass, length, and time to be recommended as the basis of the whole system, a protracted discussion has been carried on, the principal point discussed being the claims of the gramme, the metre, and tho second, as against the gramme, the centimetre, and the second,—the former combination having an advantage as regards the simplicity of the name metre, while the latter combination has the advantage of making the unit of mass practically identical with the mass of unit-volume of water-—in other words, of making the value of the density of water practically equal to unity. We are now all but unanimous in regarding this latter element of simplicity as the more important of the two; and in support of this view we desire to quote the authority of Sir W. Thomson, who has for a long time insisted very strongly upon the necessity of employing units which conform to this condition.
We accordingly recommend the general adoption of the Centimetre, the Gramme, and the Second as the three fundamental units; and until such time as special names shall be appropriated to the units of electrical and magnetic magnitude hence derived, we recommend that they be distinguished from “absolute” units otherwise derived, by the letters “C. G. S.” prefixed, these being the initial letters of the names of the three fundamental units. Special names, if short and suitable, would, in the opinion of a majority of us, be better than the provisional designations “C. G. S. unit of . . . .” Several lists of names have already been suggested; and attentive consideration will be given to any further suggestions which we may receive from persons interested in electrical nomenclature.
The “ohm,” as represented by the original standard coil, is approximately 109 C. G. S. units of resistance; the “volt” is approximately 108 C. G. S. units of electromotive force; and the “farad” is approximately 1/109 of the C. G. S. unit of capacity.
For the expression of high decimal multiples and snbmultiples, we recommend the system introduced by Mr. Stoney, a system which has already been extensively employed for electrical purposes. It consists in denoting the exponent of the power of 10, which serves as multiplier, by an appended cardinal number, if the exponent be positive, and by a prefixed ordinal number if the exponent be negative.
Thus 109 grammes constitute a gramme-nine; 1/109 of a gramme constitutes a ninth-gramme; the approximate length of a quadrant of one of the earth's meridians is a metre-seven, or a centimetre-nine.
For multiplication or division by a million, the prefixes mega* and micro may conveniently be employed, according to the present custom of electricians. Thus the megohm is a million ohms, and the microfarad is the millionth part of a farad. The prefix mega is equivalent to the affix six. The prefix micro is equivalent to the prefix sixth.
The prefixes kilo, hecto, deca, deci, centi, milli can also be employed in their usual senses before all new names of units.
As regards the name to be given to the C. G. S. unit of force, we recommend that it be a derivative of the Greek δύναμις. The form dynamy appears to be the most satisfactory to etymologists. Dynam is equally intelligible, but awkward in sound to English ears. The shorter form, dyne, though not fashioned according to strict rules of etymology, will probably be generally preferred in this country. Bearing in mind that it is desirable to construct a system with a view to its becoming international, we think that the termination of the word should for the present be left an open question. But we would earnestly request that, whichever form of the word bo employed, its meaning be strictly limited to the unit of force of the C. G. S. system—that is to say, the force which, acting upon a gramme of matter for a second, generates a velocity of a centimetre per second.
The C. G. S. unit of work is the work done by this force working through a centimetre; and we propose to denote it by some derivative of the Greek έργον. The forms ergon, ergal, and erg have been suggested; but the second of these has been used in a different sense by Clausius. In this case also we propose, for the present, to leave the termination unsettled; and we request that the word ergon, or erg, be strictly limited to the C. G. S. unit of work, or what is, for purposes of measurement, equivalent to this, the C. G. S. unit of energy, energy being measured by the amount of work which it represents.
The C. G. S. unit of power is the power of doing work at the rate of one erg per second; and the power of an engine, under given conditions of working, can be specified in ergs per second.
For rough comparison with the vulgar (and variable) units based on terrestrial gravitation, the following statement will be useful:—
The weight of a gramme, at any part of the earth's surface, is about 980 dynes, or rather less than a kilodyne.
The weight of a kilogramme is rather less than a megadyne, being about 980,000 dynes.
Conversely, the dyne is about 1.02 times the weight of a milligramme at any part of the earth's surface; and the megadyne is about 1.02 times the weight of a kilogramme.
The kilogrammetre is rather less than the ergon-eight, being about 98 million ergs.
The gramme-centimetre is rather less than the kilerg, being about 980 ergs.
For exact comparison, the value of g (the acceleration of a body falling in vacuo) at the station considered must of course be known. In the above comparisons it is taken as 980 C. G. S. units of acceleration.
One horse-power is about three quarters of an erg-ten per second. More nearly, it is 7.46 erg-nines per second; and one force-de-cheval is 7.36 erg-nines per second.
The mechanical equivalent of one gramme-degree (Centigrade) of heat is 41.6 megalergs, or 41,600,000 ergs.
* Before a vowel, either meg or megal, as euphony may suggest, may be employed instead of mega.
Mr. Stoney has requested the insertion of the following extract from one of his letters, written subsequently to the presentation of the foregoing Report:— “Would you oblige me very much by putting on record, either in the Report or as a footnote to it, that the centimetre was recommended as the unit of length against my earnest remonstrance, and that I am in no degree responsible for this decision. I would be glad to have the objections I urged against it stated also. They were, ‘that it is far too small, and that its multiples and submultiples cannot be briefly designated. From its being too small, it, in conjunction with the gramme and second, lands us in quite out-of-the-way mechanical units—the unit of force which results being but little more than the pressure of a milligramme, and the unit of work being but little more than the hundredthousandth part of a grammetre. This I deem a very serious objection.’
“I still think that these awkward consequences, and the footing which the metre has already gained in science, will prove fatal to the recommendation of the Committee, and that experience will show that the metre must in the end be accepted as the standard unit of length.”
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