I 



ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 



155 



a,t Berlin ; -when they are completed a comparison of the British Associa- 

 tion units with the mercury standards of the Reichsanstalt will become 

 possible. 



A fourth coil, constructed for the Berlin Reichsanstalt by Messrs. 

 Elliott Bros., was also tested. Table XVI. contains the results of the 

 comparison : — 



Table XVI. 



In the experiments marked thus * a considerable length, 200 cm., 

 •of the bridge wire was used. An error of 1° in the temperature of this 

 •would produce an error of 00003 in the result. In reducing the results 

 the temperature of the bridge has been taken as 18°, the value given by 

 a thermometer laid alongside the wire on July 28. In the other experi- 

 ments a coil of ] 00 ohms was put in multiple arc with the Berlin standard, 

 and the difference of the combination and the British Association units 

 was found. The length of bridge wire was very small, so that an exact 

 inowledge of its temperature was not wanted, while the 100-ohm coil 

 "was known -with all the necessary accni-acy. 



Another set of comparisons, leading to results which do not differ from 

 the above by more than -00003 British Association unit, were made by 

 Dr. Lindeck, using apparatus he had brought from Berlin ; but a 

 •complete discussion of the whole can best be given when the values of 

 the Berlin coils in terms of the mercury standard have been found. 



Dr. Lindeck also brought four Clark cells. Of these two, Nos. 69 

 and 71, were of the pattern described by Dr. Kahle (' Electrotechnische 

 Zeitschrift,' July 22, 1892; 'Zeitschrift fiir Instrumentenkunde,' April 

 1892). The mercurous sulphate is enclosed in a porous pot. The zinc 

 dips among the crystals of the zinc sulphate, its end being bent so as to 

 be horizontal. The vertical portion of the zinc is covered with a glass 

 tube, so that the horizontal part alone is effective. 



