OCTOUEK 29, 1908] 



jVA ture 



679 



It is not to be inferred from this tfiat wc know tfie 

 ohm — 10' C.G.S. units — to this accuracy in ter:ns 01 

 mercury, and the difference between the ohm and the 

 international ohm remains a matter for experiment, but 

 resistances are compared to six or even seven figures, and 

 it is requisite, therefore, for international purposes, that 

 the unit in terms of which they arc expressed should bo 

 delined with the same precision. 



So, too, with the ampere ; the definition has been 

 rendered precise by stating^ that the international ampere 

 is the current which, under, certain conditions, deposits 

 o-ooiii8oo gram of silver per second. In this case 

 decision was rendered much more difficult by the fact that 

 we know that in order to represent the ampere (10- ' 

 C.G.S. units) the last two figures should probably be 25 

 or 20. 



The following is a brief risumi of the more important 

 proceedings of the meetings. 



In discussing the general question of the measurement 

 of resistance by a mercury column. Lord Rayleigh ex- 

 pressed some doubt as to whether the introduction of 

 such a column was not what is called a " fifth wheel to 

 the coach." At the present time there was reason to 

 believe that Che ohm, as defined in absolute measure, could 

 be arrived at with a very great degree of precision. He 

 looked forward to the time when the column of jnercury 

 might be eliminated from the definition of the international 

 ohm, and when the ohm, 10° C.G.S. would bo the 

 st.andard. 



Resolution 5. defining the international ohm, wa-; 

 introduced by Dr. Warburg (Germany). In discussing it 

 Dr. Rosa (United States of America) raised the question 

 whether it would not be belter to specify the lensrth of 

 the column as i metre, and to give the weight according!} . 

 so that the resistance would be the same. The mass ol 

 mercury would then be i2.78qS grams instead of I4-j52i. 

 He suggested that if the specification for the international 

 ohm was ever changed, and it probably would be as we 

 came more nearly to the absolute value, it would be 

 necessary to change both figures if the cross-section was 

 to remain about i square millimetre. If the length we-.-e 

 specified as i metre exactly, that would never be changed. 



Dr. Rosa's suggestion was referred to the technical 

 committee, and was not approved. 



Mr. Trotter thought the resolution proposed by Dr. 

 "Warburg was something more than a confirmation of the 

 <ihm as established at Chicago. The scientific length was 

 to be set aside and a conventional length declared, like 

 the original Siemens unit. The two zeros after the 106.3 

 could have no scientific meaning. He thought the mercury 

 column was an ingenious device which would serve a 

 useful purpose, but there was no pressing need for it as 

 a standard. While a useful result of the conference would 

 be an organisation for the comparison of standards of 

 different countries, it w^as question.able whether the differ- 

 ences which had to be reconciled were of sufficient import- 

 ance to set aside the C.G.S. system and no longer to look 

 upon it as the ideal. He thought it premature to add the 

 two zeros to the 106-3, because it seemed likely that the 

 I.orenz or some other mode of determination of the ohm 

 w^ould soon be made with an accuracy within a few parts 

 in 100,000. Until that time arrived, mercury columns 

 would be a temporar}" expedient. 



Dr. Rosa, however, doubted whether the time was 

 coming when resistances could be measured absolutely as 

 accurately, and a series of determinations made to agree 

 as closely, as mercury ohms could be set up and measured. 



Dr. Warburg thought that the accuracy with which the 

 proposed international ohm aopro.ached the true ohm was 

 sufficient, and that an alteration in the international ohm 

 should not take place in the future on account of its 

 difference from the ohm (10° C.G.S.). He thought it of 

 the utmost importance to preserve continuity. 



In the end the resolution, as proposed bv Dr. War- 

 burg, was adopted. 



The next matter taken up bv the conference was the 

 Question whether the amnere or volt should be the second 

 primary unit. In openin." it. Dr. Glazebrook moved that 

 the amnere be the second Drimar\' unit. The ampere has 

 been defined by all conifresses. with the exception of that 

 of 1881, as the second prime-v unit, and ho thought that 



NO. 20,^5, VOL. 7S] 



as a standard the silver voltameter had a greater accuracy 

 of reproduction than any form of standard cell. He 

 directed attention to the following values obtained for the 

 electrochemical equivalent of silver : — 



mgin. per coulumb 



1SS4. Mascait ... .-. I'II56 



1884. V. and \V. Kohlrausch fiiSj 



1884. Riyleigh and Sidgwick ... ... I'I179 



1890. Fellat and Potitr ... IM192 



1899. Kahle ■■11S3 



1903. Pellat and Lediic . ... .. 1 iii,j 



1904. van Dijk and Kunst .. - iiibj 



19,06. Gulhe iii.-'2 



J9^7. Su.iih, Mather and Low.y .. 11183 



1905. Janet, Laporie and de I . ^imCJ 1 '118.2 



These results sho.ved that the standard could be considered 

 permanent and accurate ; and still further confirmation is 

 afforded by the fact that the Board of Trade ampere 

 balance, which was adjusted fourteen years ago on the 

 basis that the unit of current deposits 1.118 mgm. 

 of silver per second, now gives for the equivalent of silver 

 the value 1-11794 — '"i" extremely good agreement. A 

 further reason for adopting the ampere as the second 

 primary unit was that the absolute determinations cf the 

 ohm and the ampere were independent, and thus conformed 

 more nearly to the theoretical ideal. This was of import- 

 ance, as he hoped that some day we might arrive at 

 standards which would measure resistance and current in 

 absolute units direct. The chemistry of the silver volta- 

 meter had recently been inves-ijjated at the National 

 Physical Laboratory ; it w,-is of importance to know that 

 only one chemical had to be purified, and this (silver 

 nitrate) one of extreme solubility in water, and therefore 

 capable of purification by repeated crystallisation. No limi- 

 Iiad to elapse between setting up the voltameter and tile 

 attainment of a condition of chemical equilibrium. 11 

 secondary reactions took place, they were of very snutll 

 importance, while it had been shown that the temperature 

 coefficient was certainly not greater than 1 part in 

 i.ooo.ooo, and was probably smaller than this. Dr. 

 Glazebrook did not suggest that the silver voltameter 

 should be generally used for measurement of current ; it 

 was intended as an instrument to be used at standardising 

 laboratories. For all ordinary measurc-ients of current a 

 ci-'r-dard cell and a resistance would still be employed. 

 The Weston cell involved the purification of four sub- 

 stances, one of which (mercurous sulphate) was a very 

 insoluble salt and very difficult of purification ; moreover, 

 there was a difference of opinion as to the best method 

 for its preparation. If the volt were defined as a fraction 

 of the E.^LF. of the Weston cell, the standard was cer- 

 tainly more concrete, but this was not very important for 

 a standardising laboratory. Of between 300 and 400 cells 

 set up at the National Physical Laboratory 80 per cent, 

 agree w'thin three or four parts in 100,000 ; but in the 

 remaining 20 per cent, the differences may attain two in 

 10,000, and we do not know the reason of this. The results 

 obtained bv Prof. Janet, at the Laboratoire Central, showed 

 that the permanence of the cell is far from certain, as the 

 mean F.M.F. of one batch of cells dropped six parts in 

 10,000 in two years, and of another batch seven parts in 

 100,000 in one year. He agreed that an extraordinary 

 concordance in the E.M.F. of Weston cells had been 

 reached between the Bureau of Standards and the National 

 Physical Laboratory, but it was requisite to obtain agree- 

 ment with cells made at other pl.aces, and this had not yet 

 been done. 



Prof. Lippmann (France) was in favour of the volt as 

 the second primary unit. He agreed with Dr. Glazebrofik 

 that everybody would use the cell in practical work. He 

 considered that the volt was an independent unit and 

 could be measured absolutely by means of a disc rotating 

 in the earth's magnetic field. Subsequently M. Ger.-^rd 

 (Belgium) pointed out that as this method involved the 

 delermin.Ttion of the macnetic field of the earth, it was not 

 co'iTperable wilh the absolute measurement of current. 



Prof. Carhart (U.S. .A.) pointed out that the congress nf 

 iSSi proposed the volt as the seennd primary unit. In 

 Germany all measurements of E.M.F. were made by means 

 of a standard cell and resistance, and to all intents and 

 purposes the E.M.F. of the Weston cell had been leg.-iiised. 



