68o 



NA TURE 



[October 29, 1908 



ami the silver voltameter was very rarely set up. Lord 

 Kelvin at one time standardised his current balances by 

 means of the silver voltameter, but he abandoned it in 

 favour of a cell and a resistance. He (Prof. Carhart) be- 

 lieved the cell to be a constant and a useful standard. 

 With the absolute balance at the National Physical 

 Laboratory the electrochemical equivalent of silver was not 

 directly determined; it was the E.^LF. of a Weston cell 

 that was first fixed. He considered this was the correct 

 way. At the Board of Trade the balance had only been 

 compared with the silver voltameter once during the past 

 eight years. He did not consider the ageing of cells to 

 be serious ; if necessary they might be kept for only a few 

 days or a week. Cells which were set up by unskilled 

 persons should not be considered, as it was a primarv 

 standard which was under discussion. 



Dr. Rosa contended that the voltage of the Weston cell 

 should be defined and fixed ; and that, since some uncertainty 

 must be permitted in the value of its voltage or in the 

 value of the electrochemical equivalent of silver, it should 

 be in the latter, as it was infrequently used, and would 

 therefore be of minor importance. He objected to the 

 silver voltameter because it is not permanent ; it only lasts 

 so long as the current flows. It is not a concrete standard 

 like the cell, and it is not portable. It is laborious in 

 practice, and it determines electric quantity and not current. 

 Regarding the choice between Clark and Weston cells, it 

 was possible that the Clark cell was the more stable. 



Dr. Warburg thought that mercurous sulphate, which is 

 used as the depolariser in Weston cells, could not be well 

 defined, and that many cells gave abnormal results because 

 of this. In fact, mercurous sulphate had been so much 

 studied during the past three years that the Weston cell 

 of to-day was a new one. He agreed entirely with the 

 views expressed by Dr. Glazebrook. 



In the further course of the discussion the following 

 table of results for the E.>LF. of the Weston cell was 

 submitted for consideration ' : — 



EM.V. of H'ei/oi; Sorma} Cell at 20° C. 



National Bureau of Standard... | ]'°'lf '^■- <'^"' bUch of cells) 



1. I -01853 ;'. (second „ „ ) 

 National Physical Laboratory ... I '0182 v. 

 Laboratoire Cen'ral d'Electricite i'oi87 v. 



\ roi825 6'. (first group) 

 '" j roiSigz'. (second ,, ) 



Lippmann and Guillet 



.\ny uncertainty in the value of the resistance in inter- 

 national ohms w-ould, it was pointed out, naturally affect 

 these values. Ultimately the resolution in favour of the 

 ampere w^as carried by 19 votes to 4. 



Considerable discussion took place on resolution 7, 

 the definition of the second primary unit — the ampere. 

 Some of the delegates wished the ampere to be defined as 

 the unvarying current depositing silver at the rate of 

 o-ooiii8 gram per second: other delegates desired 

 O'OOiiiSoo, that is, they wished the ampere to be so 

 defined that comparisons could be made WMthin one part 

 in 100,000. The delegates from the LInited States, Dr. 

 Weber (Switzerland), and some others desired that the 

 international ampere should agree as closely as possible 

 with the ampere (10- ' C.G.S.). The values suggested were 

 0001 1 1820 or 000111S25. 



After some preliminary discussion at which this diver- 

 gence of view was made clear, the question of the exact 

 number to be inserted in resolution 7, defining the 

 ampere, was referred to the Technical Committee, and 

 discussed by them at a long sitting. The suggestion was 

 made that in the resolution the conference should be con- 

 tent to stop at the 8, but that in the specification or in the 

 notes a statement should be made as to the figures to 

 follow the 8 in measurements of precision, and this was 

 at first accepted. When, however, an attempt was made 

 to settle what these fisures should be. agreement could not 

 be reached, and ultimately it was arranged to report the 

 various votes which had been taken in committee to the 

 full sitting. Thus, when the conference took up the ques- 

 tion again, resolution 7, defining the ampere as the 

 current depositing 000111800 gram of silver per second, 

 was still before them. In the discussion which ensued, 



l_To 'his tahle must be addei the results jtist obtained ly M. Pellat, 

 which g"ve the value i'oi'?4. 



NO. 2035, VOL. 78] 



Dr. Glaz brook, who nicvtd l!ie adoption of resolution 7, 

 urged that as the object oi the conierence was to secure 

 unilormity of international measurements to a high degree 

 of precision, measurements of current to five or six figures 

 at least were wanted, and that it appeared that our units 

 must be defined to five or six figures. While the value 

 chosen should approach the absolute C.G.S. value closely 

 he did not think it essential to get as close as possible 

 to this value. In the case of the ohm, 00 had been added 

 to the 106-3 '^"i-' ^nd for consistency two more figures 

 must be added after the 8 for the electrochetnical equiva- 

 lent of silver. He would prefer to add 00. If, in the 

 future, it seemed possible to revert to the absolute units, 

 the two standards might be changed together, but so 

 long as the ohm could not be defined closely in terms of 

 the C.G.S. system there was no real necessity to define 

 the ampere closely. 



Prof. Lippmann proposed that the international ampere 

 shoud be defined as being equal to the ampere based on 

 the C.G.S. system, but this proposal was not accept^. 

 Mr. Trotter thought that nothing should be added after 

 the 8. 



Ultimately resolution 7 was carried in its original form 

 by 21 votes to 3. 



The question was again raised when the final report was 

 submitted for approval. Dr. Carhart stated that the chief 

 argument which had been brought forward in favour of 

 the ampere as the second primary unit was the repro- 

 ducibility of the silver voltaineter and the concordant results 

 obtained in determinations of the electrochemical equivalent 

 of silver in different countries. Since that discussion. Dr. 

 Rosa had received a cablegram from Washington stating 

 that the value of the electrochetnical equivalent of silver 

 obtained with the aid of a current balance at the Bureau 

 of Standards was 0001 1 182, agreeing very closely with the 

 last five determinations. Surely, if anything was to be 

 added after the 8, the figures should be 20. 



The majority of the delegates appeared to be, however, 

 of opinion that the change In the equivalent should be 

 made, if at all, at the saine time as the change in the 

 length of the mercury column, and in consequence the 

 original decision of the conference was confirmed by n 

 majority of 13 votes to 8, three countries not voting. 



Specifications relating to mercury standards of resistance 

 and to the deposition of silver were approved by the con- 

 ference and included in Schedule B, while the duty of 

 drawing up, as an appendix to the report, a series of notes 

 to the specifications, and planning more fully the inethocfs 

 to be adopted to realise the units, was assigned to a 

 scientific committee norninated by the president. 



In cases in which it is not desired to set up the standards 

 provided in the resolutions in Schedule B, the conference 

 recommends the following as working methods for the 

 realisation of the international ohm, the international 

 ampere, and the international volt. 



(i) For Die Intcruatioual Ohm. 



The use of copies, constructed of suitable material and 

 of suitable form and verified from time to time, of the 

 international ohm, its multiples and submultlples. 



(2) For flic liitcnialioiial Avipere. 



(a) The measurement of current bv the aid of a current 

 balance standardised by comparison with a silver volta- 

 meter ; or 



(h) The use of a Weston nor iial cell whose electro- 

 motive force has been determined In terms of the Inter- 

 national ohm and international ampere, and of a resistance 

 of known value in international ohms. 



(3) For the Init'niaficnaJ Volt. 



(a) A comparison with the difference of electrical poten- 

 tial between the ends of a coil of resistance of known value 

 in international ohms, when carrying a current of known 

 value in international amperes ; or 



(b) The use of a Weston normal cell whose electro- 

 motive force has been determined in terms of the inter- 

 national ohm and the international ampere. 



Steps necessary to secure Uniformity of Standards in the 

 Future. 



The conference expressed a wish that some permanent 

 steps for securing uniformity of standards in the future 

 should be taken, and for such a purpose recommended the 



