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

1

“farad” is approximately of the C.G.S. unit of capacity.

109

For the expression of high decimal multiples and sub-multiples, 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 a multiplier, by an appended cardinal number, if the exponent be positive, and by a prefixed ordinal number if the exponent be negative.

1

Thus 109 grammes constitute a gramme-nine; of a gramme

109

constitutes a ninth-gramme: the approximate length of a quadrant of one of the earth's meridians is a metre-seven, or a centimetrenine.

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 dúvaus. 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

Before a vowel, either meg or megal, as euphony may suggest, may be employed instead of mega.

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 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 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 pyov. 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 comparison 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.

Second Report of the Committee for the Selection and Nomenclature of Dynamical and Electrical Units, the Committee consisting of PROFESSOR SIR W. THOMSON, F.R.S., PROFESSOR G. C. FOSTER, F.R.S., PROFESSOR J. CLERK MAXWELL, F.R.S., G. J. STONEY, F.R.S., PROFESSOR FLEEMING JENKIN, F.R.S., DR. C. W. SIEMENS, F.R.S., F. J. BRAMWELL, F.R.S., PROFESSOR W. G. ADAMS, F.R.S., PROFESSOR BALFOUR STEWART, F.R.S., and PROFESSOR EVERETT (Secretary).

[1874.]

THE Committee on the Nomenclature of Dynamical and Electrical Units have circulated numerous copies of their last year's Report among scientific men both at home and abroad.

They believe, however, that, in order to render their recommendations fully available for science teaching and scientific work, a full and popular exposition of the whole subject of physical units is necessary, together with a collection of examples (tabular and otherwise) illustrating the application of systematic units to a variety of physical measurements. Students usually find peculiar difficulty in questions relating to units; and even the experienced scientific calculator is glad to have before him concrete examples with which to compare his own results, as a security against misapprehension or mistake.

Some members of the Committee have been preparing a small volume of illustrations of the C.G.S. system [Centimetre-GrammeSecond system] intended to meet this want.

[The first edition of the present work is the volume of illustrations here referred to.]

Resolutions adopted by the International Congress of Electricians at Paris at the sitting of September 22nd, 1881.

1. For electrical measurements, the fundamental units, the centimetre (for length), the gramme (for mass), and the second (for time), are adopted.

2. The ohm and the volt (for practical measures of resistance and electromotive force or potential) are to keep their existing definitions, 109 for the ohm, and 108 for the volt.

3. The ohm is to be represented by a column of mercury of a square millimetre section at the temperature of zero centigrade.

4. An International Commission is to be appointed to determine, for practical purposes, by fresh experiments, the length of a column of mercury of a square millimetre section which is to represent the ohm.

5. The current produced by a volt through an ohm is to be called an ampère.

6. The quantity of electricity given by an ampère in a second is to be called a coulomb.

7. The capacity defined by the condition that a coulomb charges it to the potential of a volt is to be called a farad.

217

INDEX.

The numbers refer to the pages.