ON STANDAKDS FOE USE IN ELECTRICAL MEASUREMENTS. 



149 



about -00015 ohm, and no-w has a value of •9&7860 at 10-98° and a tem- 

 perature coefficient also of "000272. 



The fact that the temperature coefficient of ^ 101 is the same as 

 that of 100 ^ would appear to show that it has now reached its per- 

 manent state. 



Messrs. Elliott Bros, possess a standard |^ 63, made at the same time 

 as the above two coils which in August 1884 had a resistance of 



1-00027 legal ohms at 13-8°, 



while in April 1886 it was found to be -99928 at 166° and 



•99992 at 18-6°. 



From this it follows that its value at 18-8° would be '99998, indicating 

 a fall of -00029 in a year and eight months. 



This coil showed marked traces of the green coloration referred to 

 in the Report, but its insulation resistance was tested and found to be 

 ■ 8000 megohms. Both the coils ^ 100 a^id 101 show slight traces of the 

 green colour ; their insulation, however, is remarkably high. It would 

 seem, then, that it is very necessary to avoid the use of newly made coils 

 in important researches, and to keep a careful check on any secular 

 changes by means of repeated comparison. We hope, when the permanence 



of ^ 101 has been certainly established, to remove the paraffin and see 

 if there is any change in the coil visible to the eye which could account 

 for this fall in resistance. 



The two ten-ohm coils '^ 102, 103 have also been compared with the 

 one-ohm in the manner described in the reports, and the values are given 

 in the tables below. These coils are stated by Messrs. Elliott Bros, to be 

 made of ' the same wire of platinum silver -015 of an inch diameter and 

 3"52 metres long.' 



Eesistance of Coil, I^ 102. 



Mean value 

 Temperature coefficient 



9-990597 legal ohms at 1283°. 

 •00289. 



This ise presented by the straight line (drawn thus 

 diagram) ^ 102, Plate V. 



on the 



