5 z THE POPULAR SCIENCE MONTHLY. 



a copy of the standard metre is kept in the Standards Department of 

 the Board of Trade, it is impossible to procure legalized rods repre- 

 senting it, and to use a non-legalized copy of a standard in commerce 

 is deemed fraudulent. Would it not be desirable that the British As- 

 sociation should endeavor to bring about the use in this country of the 

 metre and kilogramme, and, as a preliminary step, petition the Govern- 

 ment to be represented on the International Metrical Commission, 

 whose admirable establishment at Sevres possesses, independently of 

 its practical work, considerable scientific interest, as a well-found 

 laboratory for developing methods of precise measurement ? 



Next in importance to accurate measures of length, weight, and 

 time, stand, for the purposes of modern science, those of electricity. 



The remarkably clear lines separating conductors from non-con- 

 ductors of electricity, and magnetic from non-magnetic substances, 

 enable us to measure electrical quantities and effects with almost 

 mathematical precision ; and, although the ultimate nature of this, 

 the youngest scientifically investigated form of energy, is yet wrapped 

 in mystery, its laws are the most clearly established, and its measuring 

 instruments (galvanometers, electrometers, and magnetometers), are 

 among the most accurate in physical science. Nor could any branch 

 of science or industry be named in which electrical phenomena do not 

 occur, to exercise their direct and important influence. 



If, then, electricity stands foremost among the exact sciences, it 

 follows that its unit measures should be determined with the utmost 

 accuracy. Yet, twenty years ago, very little advance had been made 

 toward the adoption of a rational system. Ohm had, it is true, given 

 us the fixed relations existing between electromotive force, resistance, 

 and quantity of current ; Joule had established the dynamical equiva- 

 lent of heat and electricity; and Gauss and Weber had proposed their 

 elaborate system of absolute magnetic measurement. But these in- 

 valuable researches appeared only as isolated efforts, when, in 1862, 

 the Electric Unit Committee was appointed by the British Association, 

 at the instance of Sir William Thomson, and it is to the long-continued 

 activity of this committee that the world is indebted for a consistent 

 and practical system of measurement, which, after being modified in 

 details, received universal sanction last year by the International Elec- 

 trical Congress assembled at Paris. 



At this congress, which was attended officially by the leading 

 physicists of all civilized countries, the attempt was successfully made 

 to bring about a union between the statical system of measurement 

 that had been followed in Germany and some other countries and the 

 magnetic or dynamical system developed by the British Association, 

 also between the geometrical measure of resistance, the (Werner) 

 Siemens unit, that had been generally adopted abroad, and the British 

 Association unit, intended as a multiple of Weber's absolute unit, 



