132 



THE POPULAR SCIENCE MONTHLY, 



mcnt by a committee. In it all the units 

 of measurement arc referred to three funda- 

 mental units, the centimetre, the gramme, 

 and the secund, whence it is called the centi- 

 mctre-gramme-secund system of units (ex- 

 pressed by the symbol C. G. S.). The units 

 practically employed multiples or sub-mul- 

 tiples of the fundamental units are the 

 ohm^ or unit of resistance (symbol R.), the 

 volt^ or unit of electro-motive force (sjinbol 

 E.), and the %vehc7\ or unit of intensity (sym- 

 bol I.). Their relation to each other is ex- 



E 

 pressed by the equation, I = -, whence, tlie 



H 



value of two of the elements being known, 

 that of the other can be determined. The 

 unit of resistance, or ohm, is determined by 

 a long and complicated formula, so that it is 

 easier to get it at once by comparison with 

 the material standard which is kept at Lon- 

 don. Graduated resistance-boxes containinjr 

 electric coils carefully adjusted to the resist- 

 ance-force they are intended to represent, 

 are sold by the instrument-makers. Some 

 idea of what the ohm is may be given by 

 saying that a wire of pure copper a metre 

 (or 39^ inches) long and a milimeter in di- 

 ameter (or about ^5- of an inch) represents a 

 resistance of one fiftieth of an ohm ; conse- 

 quently, fifty metres, or one hundred and 

 fifty and a half feet of such wire, will repre- 

 sent an ohm. Common copper wire offers 

 a stronger resistance, so that only thirty or 

 forty metres of it are required to represent 

 an ohm. The volt, or unit of electro-motive 

 force, is not represented by any actual exact 

 standard, but several constant piles exist, 

 the force of which has been exactly meas- 

 ured, which may be referred to. A Daniell 

 battery, having its copper immersed in a 

 saturated solution of sulphate of copper, 

 and its zinc in a saturated solution of sul- 

 phate of zinc, has a force of 1*079 volt. The 

 electro-motive force may be measured in 

 practice by using galvanometers which are 

 graduated in volts, the exactness of which is 

 proportioned to the amount of the resistance 

 they offer. One weber represents the in- 

 tensitv of a current having a force of a volt 

 and passing over a circuit which offers an 

 ohm of resistance. The intensities of cur- 

 rents in ordinary industrial use are repre- 

 sented by fractional units of the weber, the 

 millivicbcr^ or thousandth of a weber, for tel- 



egraphic, domestic, and medical currents, 

 the microweber, or millionth of a weber, for 

 telephonic currents. Telegraphic currents 

 vary in intensity from five to twenty milli- 

 webcrs ; the currents of the Gramme ma- 

 chines that feed the Serrin regulators, of 

 from twenty to thirty webers. Some ma- 

 chines used in electrotyping afford still more 

 intense currents, often exceeding eighty we- 

 bers, although their electro-motive force is 

 very feeble. In France they sometimes 

 measure by the kilometre of resistance, 

 meaning by that the resistance which is 

 offered by a telegraphic wire four millime* 

 tres or about one sixth of an inch in di- 

 ameter, and a thousand metres or five fur- 

 longs long, which is equivalent to about ten 

 ohms. The unit of Siemens (U. S.), employed 

 in Germany, represents the elastic resist- 

 ance of a column of mercury having the 

 length of a metre and a section of a square 

 millimetre, and is equivalent to 0"9536 of an 

 ohm. Several units of intensity founded on 

 the chemical action of electric currents are 

 in use such, for example, as may be found- 

 ed on the quantity of gases disengaged in a 

 minute by a voltameter placed in a circuit, 

 or the amount of copper that may be de- 

 posited in an hour in an electrolytic bath 

 which is traversed by the current to be 

 measured. Standard apparatuses have also 

 been made, so graduated as to furnish on a 

 simple reading the intensities in webers and 

 microwebers. 



German Anthropology. The German 

 anthropologists are making a study of 

 the relative distribution of blondes and 

 brunettes in aid of their investigation of 

 the origin and ethnological composition of 

 the German people. The reports on this 

 subject, presented by Professor Virchow to 

 the recent German Anthropological Con- 

 gress, seemed to indicate the existence of 

 centers of light-colored populations in 

 Schleswig-IIolstein, the country of the low- 

 er Elbe, Hanover, and Pomerania, and of 

 dark-colored stocks in Bavaria, along the 

 Rliine, in western Belgium, and in Switzer- 

 land. No superiority over the other is as- 

 cribed to either complexion, but the differ- 

 ence is one of original stocks. The blondes 

 appear to have come down from the north- 

 cast of Europe and pressed the native dark 



