PRACTICAL STANDARDS FOR ELECTRICAL ilEASURExMENTS. 



153 



which -were graduated in tenths of a degree, each division being about 

 0-6mm. in length, so that the temperatures could not be read -with cer- 

 tainty to less than 0-02° C. If all these errors should be made in one 

 direction in making one determination of difference of resistance, and all 

 in the opposite direction in making a second, there is a possible maximum 

 error of about 3 per cent, in the value of the temperature coefficient 

 calculated from these two determinations. This is, however, highly im- 

 probable, and, moreover, makes no allowance for taking the mean of 

 several readings. The error in the temperature coefficient is probably 

 not greater than 1 per cent., if as great. 



The following summarises the results of the experiments : — 



Te7i-0hm Standard Coil, No. 3873. 



A series of tests was made on this coil in the manner above described, 

 lasting from March 22 to April 1, 1898. Determinations were obtained 

 of the difference between the resistance of No. 3873 at six different 

 temperatures, and the resistance of No. 3874 at 16-70° C, with the 

 following results : — 



From readings a, b, c, and e, and from the measurement of the resist- 

 ance of the coil made by Mr. Glazebrook in December, 1897, giving 

 Rj3.3c,=9-9901 ohms, we get 



R,=9-9398 (1 + 0-000397« - 0-000002(4)<2). 



After testing this coil the other coil (No. 3874) was tested, and then 

 three check tests were made on this coil with the following results : — 



These points lie well on the curve obtained in the former tests (see 

 fig. 5). From the formula given above the coil will have the correct 

 resistance of 10 ohms at 17-0° C. 



To compare the temperature coefficient here obtained with those 



