14 



SCIENCE. 



[X. 



Vol. I. Xo. 1. 



and marked A, deposits silver at the rate 

 of 0.001118 of a gramme per second. 



" Third. The Volt, which has the value 

 of 10* in terras of the centimetre, the 

 gramme and the second of time, being the 

 electrical pressure that if steadilj^ applied 

 to a conductor whose resistance is one ohm 

 will produce a current of one ampere, and 

 which is represented by .6974 {,\l%%) of the 

 electrical pressure at a temperature of fifteen 

 degrees C. between the poles of the voltaic 

 cell known as Clark's cell, set up in accord- 

 ance with the specification appended hereto 

 and marked B." 



The specifications referred to in the above 

 as marked A are those that were adopted 

 at the Chicago Congress, together with some 

 additional suggestions as to the methods of 

 procedure. 



The specification marked B refers to 

 the method of preparation of Clark's cell, 

 including a detailed statement as to ma- 

 terials and as to the method of setting up 

 the cells. These specifications are made 

 so as to include several different kinds of 

 cells, so that the Lord Rayleigh modifica- 

 tion of the Clark cell, and also a modifica- 

 tion devised and used by the Germans, may 

 be used at will. There is certainly a de- 

 cided advantage in this. Attached to the 

 ' Order in Council ' is a schedule which is 

 declared to set forth the several denomina- 

 tions of electi-ical standards as approved by 

 the Queen. In this schedule the standard 

 of electrical resistance is described as be- 

 ing the resistance between the coi^per ter- 

 minals of a particular coil of wire under 

 standard conditions. The standard of cur- 

 rent is described as being the current which 

 when passed thi-ough the coils forming a 

 part of a particular instrument under spe- 

 cific conditions gives rise to forces which 

 are exactly balanced bj^ the force of gravity 

 at Westnlinster upon a particular mass of 

 matter forming a part of said instrument. 

 The standard of electro-motive force, or. 



as it is termed in the ' Order in Council,' 

 ' electrical pressure,' which is denominated 

 as one volt, is described as being yjjj part of 

 the pressure which when applied between 

 the terminals of a particular instrument 

 causes the rotation of a certain portion of 

 said instrument to the extent wliich is 

 measm-ed \>j the coincidence of a certain 

 wii-e with the image in the eyepiece of the 

 telescope and with certain fiducial marks. 



A careful examination of the above defi- 

 nitions,together with the schedule following, 

 and a comparison of the same with the 

 units as defined by Act of Congress, which 

 are essentially those of the Chicago Cham- 

 ber of Delegates, will give rise to many in- 

 teresting and important reflections to which 

 space cannot now be given. It maj^ be 

 suggested, however, that there is room for 

 uncertainty under the provisions of the 

 English regulations as to what is the' 

 standard of resistance, or of current, or of 

 electro-motive force. Of course this will 

 all turn upon what would be the action of 

 the English authorities in case of a sus- 

 pected error in the material representation 

 of these standards as provided for in the 

 schedule. The ' Order in Council ' makes 

 no provision for a course of procedure in 

 such an event, and it is but natural to as- 

 sume that standards of a very complicated 

 character, and so composite in material as 

 those thus adopted, must be continually 

 liable to changes, and the reintroduction of 

 errors of considerable magnitude. 



The actual material representations of 

 these three electrical units, it will be ob- 

 served, are by this ' Order ' removed at a 

 considerable distance fi-om the ftmdamental 

 definitions adopted by the English com- 

 mittee, as well as by the Chicago Chamber 

 of Delegates, thus, although the ohm is 

 defined primarily by reference to the C. G. 

 S. system of units, and secondarily by refer- 

 ence to the column of mercurj^, in actual 

 practice it is neither the one nor the other 



