12 



SCIENCE. 



[X. S. Vol. I. No. 1. 



The diffei-euces between the definitions 

 adopted by the International Congress at 

 Chicago and tliose found in this law are 

 very sliglit, and consist entirely of verbal 

 changes that were thought to be desirable 

 and necessary by the Senate Committee to 

 ■which this bill was referred after its passage 

 hy the House of Eepresentatives. It may 

 be well to remark tliat a subcommittee of 

 the Chamber of Delegates, consisting of von 

 Helmholtz, Professor AjTton and Professor 

 Carhart, had been appointed to prepare 

 specifications for the better realization of 

 the adopted material representation of the 

 volt. The continued illness of von Helm- 

 holtz, from the time of his leaving this 

 country, at the close of this Congress, up to 

 the day of his lamented death, about a year 

 later, prevented the completion of the labors 

 of this committee at an earlier date ; how- 

 ever, correspondence had been begun, and 

 many points had been defined and settled 

 among its members. The si^ecifications for 

 the better representation of the ampere to 

 which the Chamber of Delegates had agreed 

 will be found in the report of the American 

 delegates to the Secretary of State. As 

 this subcommittee had not yet been able to 

 formulate a report, and as it was necessary 

 for Congress to make some reference to 

 these specifications in the Act adopting the 

 units, it was agreed that the matter should 

 be referred to the National Academj"- of 

 Science, as is provided in the last section of 

 the Act. This Act, as it finally became a 

 law. is as follows : 



(Public No. 105.) 



All Arl III ileflne and cstaUixh the iiniis of cicctvical 

 measure. 



Be it enacted by the Senate and House of Repre- 

 sentatives of the United States of America in Congress 

 assembled, Tliat from and after the passage of this 

 Act the legal units of electrical measure in the United 

 Stiites shall be as follows: 



Firxt. The unit of resistance shall be what Is known 

 as the international ohm, which is substantially equal 

 to oue thousand million units of resistance of the 



centimetre-gramme-secoud system of electro-magnetic 

 units, and represented by the resistance offered to an 

 unvarying electric current by a column of mercury at 

 the temperature of melting ice fourteen and four 

 thousand five bundled and frsvcntj-one ten thous;indtlis 

 grammes in mass, of a constant cross sectional area, 

 and of the length of one hundred and six and three 

 tenths ceutimeti-es. 



Second. Tlie unit of current shall be what is kno-mi 

 as the international ampere, which is oue-tenth of the 

 unit of current of the centimetre-gramme-second 

 system of electro-magnetic units, and is the practical 

 equivalent of the unvarying current, which, when 

 passed through a solution of nitrate of silver in water 

 in accordance with standard specifications, deposits 

 silver at the rate of one thousand oue hundred and 

 eighteen millionths of a gramme per second. 



Third. Tlie unit of electro-motive force shall be 

 what is knowTi as the international volt, which is the 

 electro-motive force that, steadily applied to a con- 

 ductor whose resistance is one international ohm, will 

 produce a current of an international ampere, and is 

 practically equivalent to one thousand fourteen hun- 

 dred and thirty-fourths of the electro-motive force be- 

 tween the poles or electrodes of the voltaic cell known 

 as Clark's cell, at a temperature of fifteen degrees 

 centigrade, and prepared in the manner described in 

 the standard specifications. 



Fourth. Tlie unit of quantity shall be what is 

 known as the international coulomb, which is the 

 quantity of electricity transferred by a cun-ent of one 

 international ampere in one second. 



Fifth. The unit of capacity shall be what is kno^NTi 

 as the international farad, which is the capacity of a 

 condenser charged to a potential of one international 

 volt by one international coulomb of electricity'. 



Sixth. The unit of work shall] be the joule, which 

 is equal to ten million units of work in the centi- 

 metre-gramme-second system, and which is practically 

 equivalent to the energy expended in one second by 

 an international ampere in an international ohm. 



Serenth. The unit of power shall be the watt, 

 which is equal to ten million units of power in the 

 centinietre-gramme-second system, and which is prac- 

 ticallj' equivalent to the work done at the rate of one 

 joule per second. 



Eighth. Tlie unit of induction shall be the henry, 

 which is the induction in a circuit when the electro- 

 motive force induced in this circuit is oue interna- 

 tional volt while the inducing current varies at the 

 rate of one ampere per second. 



Sec. 2. Tliat it shall be the duty of the National 

 Academj' of Sciences to prescribe and publish, as soon 

 as pos.sible after the passage of this Act, such specifica- 

 tions of details as shall be necessary for the practical 



