PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 35 



tion with one of those of the Board of Trade ; ^ these coils show no 

 difference. 



The above statements are made on the assumption that the variotig 

 changes in the coils which have undoubtedly occurred have been rightly 

 interpreted, so that we can now recover the absolute C.G.S. value of the 

 coil Flat, and hence of the standard ohm as originally determined at the 

 Cavendish Laboratory, and defined by the Committee in the Edinburgh 

 Report, 1892. 



That this is the case is borne out by the results of the experiments on 

 the specific resistance of mercury, a summary of which is given in 

 Appendix II. These are not yet complete. Mr. Smith has, however, 

 constructed and calibrated eleven mercury tubes. The mean cross - 

 section of each of these has been determined by at least four different 

 sets of measurements. In nine cases the greatest difference between any 

 measurement and the mean is not more than "001 per cent. 



The values found for the resistance of each tube do not differ by 

 more than '001 per cent. 



If we assume as above that the values of the wire standards of resist- 

 ance of the Association are known in terms of the absolute C.G S. unit, 

 then it follows that the length of the column of mercury, one square 

 millimetre in section, which would have a resistance of lO'-* C.G.S. units 

 would be 106-291 centimetres. The value found for this same quantity 

 by the Secretary (Mr. Glazebrook) and Mr. Fitzpatrick in 1888,^ was 

 106-29 centimetres. We infer then that we still can recover from our 

 standard coils the absolute C.G.S. unit of resistance. 



Again, the length of the mercury column constituting the international 

 ohm has been defined as 106-3 cm. 



But we have seen that the absolute C.G.S. unit as deduced from the 

 wire coils of the Association has a resistance equal to that of 106-291 cm. 

 Thus the absolute unit ^ is smaller than the international ohm by -009 per 

 cent. Again, it has been stated above that the unit deduced from the 

 standards of the Association is smaller than that of the Reichsanstalt by 

 •OlOfi per cent. 



Thus the mercury standards of the Reichsanstalt, constructed to repre- 

 sent the international ohm, exceed those just made for the Association 

 by Mr. Smith by "OOlj per cent., or 1-5 parts in 100,000. 



Again, if these results be accepted, since the Board of Trade unit, as 

 derived from the wire standards, is less than that of the Association by 

 •006 per cent., and the Association unit is too small by -009 per cent., it 

 follows that the Board of Trade unit is too small by -015 per cent. This 

 difference arises in part from the fact that the standards of the Association, 

 from which the Board of Trade standard was copied by the Secretary in 

 1891, are too low ; in part from the fact that the Board of Trade standard 

 has diverged slightly from that of the Association since 1891. 



' If the view be accepted that the laboratory unit is the same as in 1891, the 

 Board of Trade standard has fallen since that date by -00006 ohm 



» Phil. Trans. 1888. 



' The resistance taken for a column of mercury 1 square mm. in section. 100 cm. 

 in length at 0° C. at the Edinburgh Meeting in 181)2, was -9407 x 10" C.G.S. units. Mr". 

 Smith's experiments give, assuming the values of the wire coils known, the result 

 •9408 X 10' C.G.S. units. 



