216 Mr. F. W. Burstall on the Use of 



gauge-coil is then removed and the copper har substituted ; 

 the slider is then moved until a new balance is obtained. Pro- 

 ceeding thus, the whole wire is divided into a number of parts 

 the resistances of which are equal. From these observations 

 a chart can be prepared showing the deviation in resistance 

 of the wire from a wire of uniform resistance per unit of 

 length. One of the great advantages of this method is that, 

 when an abnormal value has been obtained, it is a simple 

 matter to come back to that particular portion of the wire 

 for a check measurement. The gauge-coil was equal to the 

 resistance of 22*293 mean scale-divisions (each 0*5 millim.), 

 and the greatest error at any point of the wire amounted to 

 *4 of a scale-division. Throughout the whole of the com- 

 parison of the coils this calibration of the wire has been 

 employed, and several apparently discrepant observations have 

 been thus brought into line. 



The temperature-coefficient of the bridge- wire has been 

 found by raising the temperature of the room from 10 o, 4 to 

 18°'l, the bridge having in one arm a coil of known tem- 

 perature-coefficient, and was found to be '071 per cent, per 

 degree. 



The thermometer employed, which is of hard Jena glass, is 

 divided into tenths of a degree, each tenth being approxi- 

 mately *5 millim. in length. By means of a small reading 

 telescope it can with ease be read to *01 of a degree. It was 

 tested at the Reichsanstalt and, over the range employed, was 

 accurate to at least *05 of a degree. 



The temperature-coefficient of the platinum-silver wire, 

 over a range from 10° to 25°, was determined by immersing 

 a coil of about 1 ohm in paraffin-oil at 25°, and allowing 

 it to cool very slowly. It was found to be '000274 per ohm 

 per degree. 



The 1 ohm coil from which the standards have been built 

 up is of manganin, and when tested in June 1894 by Mr. 

 Glazebrook had a resistance of 1*00026 ohms at J3°'4. I 

 was, however, so doubtful of the constancy of this standard 

 that in December 1895 I constructed a bare platinum-silver 

 1 ohm coil. This was compared with the manganin standard, 

 and the mean of eleven determinations gave its value as 



1-00000 + -000274 (t- 14-96). 



No doubt the manganin coil had changed its value in the 

 eighteen months that had elapsed since it was tested, so that 

 the above figures were not true ohms. 



The 10 ohm standard was obtained by the use of three 



