xl 



UNITS. 



3' 



Metric Measures and Weights. 



As explained in section i above, the standards of length and mass in the 

 metric system are the metre and the kilogramme. Two material representatives 

 of each of these standards are possessed by the United States and preserved at 

 the Office of Standard Weights and Measures at Washington, D. C. 



The standards of length are Prototype Metres Nos. 21 and 27. These are 

 platinum iridium bars of X cross section, and their lengths are defined by lines 

 ruled on their neutral surfaces. Their lengths at any temperature / Centigrade 

 are given by the following equations : — 



Prototype No. 21 =: i'" -\- 2.^*5 -|- 8.^^665 / -j- c^oo 100 /-, 

 Prototype No. 27 = i'" — 1.^6 -f- S.f^ 657 / -}- o.'^oo 100 i% 



where the symbol /a stands for one micron, or one millionth of a metre. The 

 probable errors of these Prototypes may be taken as not exceeding ± o.'^2, or 

 1/5 000000th of a metre for temperatures between 0° and 30° C. 



The standards of mass are Prototype Kilogrammes Nos. 4 and 20. They are 

 cylindrical masses of platinum iridium. Their masses and volumes are given by 

 the following equations : — 



Mass. Volume. 



Prototype Kilogramme No. 4 = 1^' — o.'"''o75, 46."''4i8, 

 Prototype Kilogramme No. 20 = i'" — o."'^o39, 46."''402, 



where the — 



Symbol ^'g stands for one kilogramme, 



Symbol mg stands for one milligramme = o.^'^oooooi, 



Symbol ;;z/ stands for one millilitre = one cubic centimetre. 



The definitive probable error assigned to the Prototype Kilogrammes by the 

 International Bureau is ± o."'''oo2, or 1/500 000 000th of a kilogramme. 



The act of Congress approved July 28, 1866, authorizing the use of the metric 

 system in the United States, provides that the tables in a schedule annexed shall 

 be recognized " as establishing, in terms of the weights and measures now in use 

 in the United States, the equivalents of the weights and measures expressed 

 therein in terms of the metric system ; and said tables may be lawfully used for 

 computing, determining, and expressing, in customary weights and measures, the 

 weights and measures of the metric system." The following copy of that sched- 

 ule gives the denominations of the multiples and sub-multiples of the measures 

 of length, surface, capacity, and weight in the metric system as well as their 

 legalized equivalents in British units. 



