150 REPORT— 1892. 



such cells is about -i- QQm ^tbs of a volt higher than that of the H cells. 

 The agreement of different cells of this form is very sufficient for practical 

 purposes ; the difference between the E.M.F. is always smaller than 

 T o 6 ^^ of a volt. The only disadvantage of this form is that its E.M.F. 

 does not follow quickly the alterations of temperature ; but I suppose one 

 could improve that by diminishing the size of the cell. 



We endeavoured in the Reichsanstalt to make Lord Rayleigh's H form 

 transportable on account of its good qualities, and to do this without 

 introducing foreign substances as porous walls. The investigations in 

 this direction are not yet finished, but I am able to exhibit a cell which 

 was constructed for this purpose and seems to be good. The positive 

 electrode is formed by an amalgamated platinum plate fixed on a wire of 

 the same metal, which is melted in the bottom of one of the two tubes 

 forming the vessel. The negative electrode is formed by the 10 per- 

 centage zinc amalgam solidified on the bottom of the other tube, and 

 also connected with a platinum wire melted in the glass. The whole 

 vessel is filled with paste and closed by a glass .stopper perforated by a 

 thermometer, of which the bulb is within the vessel. Such a cell can be 

 turned without any danger, and is suitable for transport. The E.M.F. 

 is about 100 00 ^^ o^ ^ ^^^^ smaller than that of the H cells ; the dis- 

 agreement between the E.M.F. 's of different cells constructed in similar 

 manner does not exceed -j-y^f^y^ths of a volt. But before using such cells 

 for practical purposes, they must be observed for a longer time till one 

 is sure that their behaviour will not be altered by age. 



These are the principal results obtained recently in the Reichsanstalt 

 on this matter. Some of them will be already known here ; but I 

 hope to have given new proof that the Clark cell is a very accurate 

 standard for E.M.F., and a good measuring instrument for practical 

 purposes. 



APPENDIX VI. 



On the Values of certain Standard Resistance Coils. 

 By R. T. Glazebrook, F.R.8. 



In the report of the Committee for 1800 it was stated that during the 

 early part of the year small changes had taken place in the values of some 

 of the old B.A. standards, which had been subjected to a very low temper- 

 ature early in that year. These coils have been compared together at 

 intervals since that date, and the following tables will show that at the 

 temperatures given there is no indication of any further change. The 

 difference between the coils and the standard coil flat is given in bridge 

 wire divisions. The value of one bridge wire division is about "00005 

 ohm. 



The first few lines in Tables X. and XI. give the values of the differ- 

 ences observed in 1890. These are followed by those of the more recent 

 observations which were taken at a temperature of about 16°. 



These observations are sufiBcient to show that there has been no appre- 

 ciable change in the relative values of these coils. 



The observations on the legal ohm standards given in Table XL 

 lead to the same conclusion. 



