i8 



NA TURE 



\Nov. 6, 1873 I 



connected with industrial exhibitions. The'establishment starts 

 with acapital of more than 11,500/. 



On Sept. I, an earthquake took place at 4.10 p.m. with 

 slight shocks at Drama, in European Turkey. There was an 

 earthquake at about 9 P.M. on Sept. 6, in Armenia, at Erze- 

 roum, and elsewhere. Several shocks of earthquake were felt 

 on Aug. 21, in the City of Guatemela, but very few houses 

 were damaged. 



Za Nature records the recent death of J\I. Godard, senior, 

 the oldest of French aeronauts. 



The additions to the Zoological Society's collection during the 

 past week include a Bosman's Potto {Pcrodidicus potto) \ from 

 Africa, and a Blue Magpie (Q-aHu/"^"" '">''""") '"'^"^ China, pre- 

 sented by Rev. A. W. Beter ; two Ursine Dasyures [Dasyitrus 

 iirsiniii) from Australia, presented by the Acclimatisation Soci- 

 ety of Victoria ; an Alpine Marmot {Arctomys miirnwtta), an 

 Inconvenient Curassow {Ciax incommoda) from S. America, a 

 Red-bellied Thrush (Tiirdus ru/iveiitns), a Red Oven-bird 

 (Fm-iiaritis nifiis), and two Yellow Trupials {Xanihosomus flavns) 

 from Buenos Ayres ; a Hoffmann's Sloth (Cholopiis hoffmanni) 

 from Panama, purchased ; a Sun Bittern (Euiypyga helias) from 

 S. America, deposited. 



THE SELECTION AND NOMENCLATURE OF 

 DYNAMICAL AND ELECTRICAL UNITS* 



WE consider that the most urgent portion of the task entrusted 

 to us is that which concerns the selection and nomencla- 

 ture 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 fun- 

 damental 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 wiiole system 

 of fundamental units of force; work, electrostatic, and electromag- 

 netic 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 denot- 

 ing decimal multiples and sub-multiples, 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 lias been carried on, the principal point 

 discussed being the claims of the gramme, the iiidrc and the 

 second, as .igainst the gramme, the ccutimttye, and the second ; 

 the former combination having an advant.age 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, 



* First Report of the Britisli Association Committee on Units. 



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 centi- 

 metre, 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 timt 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 

 most of us, be better th.an 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 

 inye^ectrical nomenclature. 



Tlie "ohm," as represented by the original standard coil, is 

 approximately 10'' C. G. S. units of resistance. The " volt" is 

 approximately 10' C. G. S. units of electromotive force, and 



the "farad" is aiiijroximately — -of the C. G. S. unit of 



IO-' 



capacity. 



For the expression of high decimal multiples and sub-multiples, 

 we recommend the system introduced by Mr. G J. Stoney — a 

 system which has already been extensively employed for elec- 

 trical purposes. It consists in denoting llie 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 : — 



10' grammes constitute 3. gr(. 



— 5 of a gramme constitutes a ninth-graiiune. 



The earth's circumference is approximately four metre-sevens, 

 or four centimetre-nines. 



For multiplication or division by a million, the prefixes mega * 

 and micro may conveniently be employed, according to the 

 present cu'tom of electricians. Thus the megoinn is a million 

 ohms, and the inlrrofarad 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, hccto, deca, 

 dcci, ccnti, inilli 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 Si/faiui;. 

 The form dynamy appears to be the most satisfactory to etymo- 

 logists. Dynain 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 earnestly request that, which- 

 ever form of the word be 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 work done by this force, working through a centimetre, 

 is the C. G. S. unit of work, and we propose to denote by it some 

 derivative of the Greek ip-^ov. The forms ergon, crgal, 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 

 rale 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 9S0 dynes, or rather less than a kilodyne. 



Tlie weight of a kilogramme is rather less than a megadyne, 

 being about 980,000 dynes. 



Conversely, the dyne is about i '02 times the weight of a milli- 



* Before .t vowel, cither titeg or inegal (as euphony may suggest), may 

 employed instead of mega. 



I 



