ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 135 



practically insoluble in water : its presence at any rate in moderate quan- 

 tities has no effect on the cell. If, bow ever, it is formed the acid sulphate 

 is formed also. This is soluble in water and the acid produced affects 

 the electro-motive force. The object of the washings is to dissolve and 

 remoVfe this acid sulphate, and for this purpose the three washings 

 described in the specification will in nearly all cases suffice. If, however, 

 a great deal of the turpeth mineral is formed it shows that there is a 

 great deal of the acid sulphate present, and it will then be wiser to obtain 

 a fresh sample of mercurous sulphate rather than to try by repeated 

 washings to o'et rid of all the acid. 



The free mercury helps in the process of removing the acid, for the 

 acid mercuric sulphate attacks it, forming mercurous sulphate and acid 

 which is washed away. 



The cell may be sealed in a more permanent manner by coating the 

 marine glue, when it is set, with a solution of sodium silicate and leaving 

 it to harden. 



Appendix. 



Av!/vd 12, 1892. 



Dear Sir, — T am desired by the Electrical Standards Committee of 

 the British Association to communicate to the Electrical Standards Com- 

 mittee of the Board of Trade the enclosed extract from their report 

 made to the Association on August 9, 1892. 



I remain, yours faithfully, 



(Signed) R. T. Glazebrook, 



Secretary, Electrical Standards Committee 



of the British Association. 



To Sir Thomas Blomefield, 



Secretary, Electrical Standards Committee 



of the Board of Trade. 



Extract from the Report of the Electrical Standards Committee of 

 THE Association, August 9, 1892. 



The following resolutions were agreed to : — • 



1. That the resistance of a specified column of mercui'y be adopted as 

 the practical unit of resistance. 



2. That 14"4521 grammes of mercury in the form of a column of 

 uniform cross-section 106"3 centimetres in length at 0° C. be the specified 

 column. 



3. That standards in mercury or solid metal having the same resist- 

 ance as this column be made and deposited as standards of resistance for 

 industrial purposes. 



4. That such standards be periodically compared with each other, 

 and also that their values be redetermined at intervals in terms of a 

 freshly set-up column of mercury. 



It was further agreed that these resolutions be communicated to the 

 Electrical Standards Committee of the Board of Trade. 



With regard to the units of current and electro-motive force it was 

 agreed that the number -001118 should be adopted as the number of 

 grammes of silver deposited per second from a neutral solution of nitrate 

 of silver by a current of 1 ampere, and the value 1'434 as the electro- 

 motive force in volts of a Clark cell at 15° C. 



