May 24, 1883] 



NATURE 



9i 



which commence as far back as the end of the fifteenth century, 

 the various expeditions sent out by private enterprise, those 

 despatched for military, naval, or diplomatic purposes, or, finally, 

 the various hydrographic or geodetic surveys undertaken by the 

 French authorities in Cochin China. 



The teachers at the school for the sons of Japanese nobles in 

 Tokio appear to have hit upon a notable method of teaching 

 physical geography. In the court behind the school building is 

 a physical map of the country, between three and four hundred 

 feet long. It is made of turf and rock, and is bordered with 

 pebbles, which look at a little di.-tance much like water. Every 

 inlet, river, and mountain is reproduced in this model with a 

 fidelity to detail which is wonderful. Latitude and longitude 

 are indicated by telegraph wire-, and tablets show the position 

 of the cities. Ingenious devices are employed in illustrating 

 botanical studies also. For example, the pine is illustrated by 

 a picture showing the cone, leaf, and dissected flower, set in a 

 frame which shows the bark and longitudinal and transverse 

 sections of the wood. 



In No. 103 of the Zeitschaft of the Berlin Geographical 

 Society will be found a fine series of new large scale maps by 

 H. Kiepert on the region containing the ruins of Babylon, embody- 

 ing the results 1 if new surveys and explorations. In the same 

 number Herr Karl Schneider has a long paper on the valley 

 formations of .he Eifel. 



Prof. Fries has written an interesting paper proposing that 

 part of Greenland should be colonised by Lapps. He maintains 

 that the country would be a paradise to the mountain Lapps, 

 that it is no more inhospitable than their own country, that there 

 would be no restrictions to their wanderings, and that in the 

 interior in summer and on the coast in winter they would find 

 abundant forage for their herds. Prof. Fries is of Nordenskjold's 

 opinion, that in the interior abundant reindeer pasture wdl be 

 found. Moreover, as .1 Lapp cm always follow where a reindeer 

 leads, this would be an excellent plan of discovering the true 

 nature of the interior ; it seems certainly worth trying. 



Two gentlemen from Minister (Westphalia) — Dr. Bachmann 

 and Dr. Frie.lrich Wilms — are about to start on a scientific tour 

 to Southern Africa, the Transvaal to begin with, in order to 

 make zoological and botanical researches. Their journey will 

 extend over several years, and the travellers will endeavour to 

 establish direct commercial relations between the South African 

 colonies and Germany. 



ELECTRICAL UNITS OF MEASUREMENT '» 

 ""THE lecturer began by observing that no real advance could 

 ■*■ be made in any branch of physical science until practical 

 methods for a numerical reckoning of phenomena were esta- 

 blished. The " scale of hardness " for stones and metals used 

 by mineralogists and engineers was alluded to as a mere test in 

 order of merit in respect to a little understood quality, regarding 

 which no scientific principle constituting a foundation for defi- 

 nite measurement had been discovered. Indeed it must be 

 confessed that the science of strength of materials, so all impor- 

 tant in engineering, is but little advanced, and the part of it 

 relating to the quality known as hardness lea t of all. 



In the last century Cavendish and Coulomb made the first 

 advances towards a system of measurement in electrical science, 

 and rapid progress towards a complete foundation of the system 

 was effected by Ampere, Poisson, Gre;n, Gauss, and others. As 

 late as ten years ago, however, regular and systematic measure- 

 ment in electrical science was almost unknoan in the chief 

 physical laboratories of the world ; although as early as 1858 a 

 practical beginning of systematic electric measurement had 

 been introduced in the testing of submarine telegraph cables. 



A few years have sufficed to change all this, and at this time 

 electric measurements are of daily occurrence, n it in our scien- 

 tific laboratories only, but also in our workshops and factories 

 where is carried on the manufacture of electric and telegraphic 

 apparatu-. Thus ohms, vults, amperes, coulombs, and micro- 

 farads are now common terms, and measurements in these units 

 are commonly practised to within one per cent, of accuracy. It 

 seems, indeed, as if the commercial requirements of the applica- 

 tion of electricity to lighting and other u-es of everyday life 



1 Abstract of lecture on " Electrical Units of Measurement," by Sir 

 William ihomson, F.R.SS.L. and E., M. Inst. C.E.. delivered on Thurs- 

 day evening, May 3, 1883, at the Institution of Civil Engineers. 



were destined to influence the higher region of scientific investi- 

 gation with a second impulse .ot less important than that given 

 thirty years ago by the rei .ements of submarine telegraphy. 



A first step toward the numerical reckoning of properties of 

 matter is the discovery of a continuously varying action of some 

 kind, and the means of observing and measuring it in terms of 

 some arbitrary unit or scale division ; while the second step is 

 neces-arily that of fixing on something absolutely definite as the 

 unit of reckoning. 



A short historical sketch was given of the development of 

 scientific measurement, as applied to electricity and magnetism, 

 from its beginning with Cavendish about 100 years ago, to the 

 adoption of the absolute system of measurement by this country 

 in 1869, at the instance of the Briti-h Association Committee 

 on Electric Standards, The importance in this development of 

 the originating works of Gauss and Weber was pointed out, as 

 also of the eight years' labours of the British Association Com- 

 mittee. This Committee not only fairly launched the absolute 

 system for general use, but also effected arrangements for the 

 supply of standards for resistance coils, in terms of a unit, to be 

 as nearly as possible io 9 centimetres per second. This unit after- 

 wards received the name of the ohm, which was adopted from a 

 highly suggestive paper which had been communicated to the 

 British Association in 1861 by Mr. Latimer Clark and Sir 

 Charles Bright, in which some very valuable scientific methods 

 and principles of electric measurement were given, and a system 

 of nomenclature — ohmas, kilohmas, farads, kilofarads, volts, and 

 kilovolts — now univer-ally adopted witn only unessential modi- 

 fication, was proposed for a complete system of interdependent 

 electric writs of measurement. At the International Conference 

 for the Determination of Electrical Units held at Paris in 1882, 

 the absolute system was accepted by France, Germany, and the 

 other European countries ; and Clark and Bright's nomenclature 

 was adopted in principle and extended. 



Gauss's principle of absolute measurement for magnetism and 

 electricity is merely an extension of the astronomer's method of 

 reckoning mass in terms of what may be called the universal 

 gravitation unit of matter, and the reckoning of force, according 

 to which the unit of force is that force which, acting on unit of 

 mass for unit of time, generates a velocity equal to the unit of 

 vel icity. The universal-gravitation unit of mass is such a 

 qviantity of matter, that if two quantities, each equal to it, be 

 placed at unit distance apart, the force between them is unity. 



Here mass is defined in terms of force and space, and in the 

 preceding definition force was defined in terms of mass, space, 

 and time. Eliminating mass between the two, it will be found 

 that any given force is numerically equal to the fourth power of 

 the velocity with which any mass whatever must revolve round 

 an equal mass, fixed at such a distance from it as to attract it 

 with a force equal to the given force. And, eliminating force 

 between the two primitive definitions of the universal gravitation 

 system, it will be found that any given mass is numerically equal 

 to the square of the velocity with which a free particle must 

 move to revolve round it in a circle of any radius, multiplied by 

 this radius. Thus, take a centimetre as the unit of length, 

 and a mean solar second as the unit of time, and adopt 5-67 

 grammes per cubic centimetre as the mean density of the earth 

 from Rally's repetition of Cavendish's experiment, and suppose 

 the length of th* seconds' pendulum to be 100 centimetres, and 

 neglect the oblateness of the earth and the centrifugal force of 

 its rotation (being at the equator only 1/289 of gravity), the result 

 for the universal gravitation units "f mass and force is respec- 

 tively 1 5 36 French tons, and 15 '36 megadynes, or 15-07 times 

 the terrestrial surface- weight of a kilogram. 



The ultimate principles of scientific measurement were illus- 

 trated by the ideal case of a traveller through the universe who 

 has brought with him on his tour no weights, no measures, 

 no w atch or chronometer, nor any standard vibrator or spring 

 balance, but merely Everett's units and constants, and a complete 

 memory and understanding of its contents, and who de-ires to 

 make for himself a me'rical sy-tem agreeing with that which he 

 left behind him on the e.irth. To rec >ver his centimetre the 

 readiest and most accurate way is to find how many wave- 

 lengths of sodium light there are in the distance from bar to bar 

 of a grating which he can engrave for himself on a piece of 

 glass. How easily this is done, suppo ing the grating once 

 made, was illustrated by a rapid experiment performed in the 

 course of the lecture, without other apparatus than a little piece 

 of glass with 250 fine parallel lines engraved on it by a diamond, 

 and two candles and a measuring tape of unknown divisions of 



