PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 213 



and the outside diameter of the coil so wound was measured in the 

 Whitworth machine with the foUov.'ing results : — 



General mean = 21-04818 inches. 



The temperature, which was taken at each observation, varied between 

 19°-9 C. and 21° C, and had a mean value of 20°-4 C. Correcting for the 

 difference between the tempeiature at which the bars of the Whitworth 

 machine have their specified value and this mean temperature, we have 

 for the mean outside diameter of the coil, when wound with bare wire 

 0-02136 inch thick, 



21-04932 inch at 20°-4C. 



From the above measurements it is clear that the wire lay on a very 

 true circular cylinder. With bare wire, however, of the thickness used it 

 was found impossible to obtain sufficient insulation between pairs of 

 convolutions. Hence, after much time had been spent in endeavouring to 

 insulate the successive turns by forcing paraffin wax in between them, &c., 

 the coil was unwound and rewound with double silk covered wire which 

 had been first dried, then drawn through paraffin wax, and lastly baked 

 before the winding was commenced. To wind so large and heavy a ring 

 was not an easy matter, and it was not until the winding had been 

 performed three times that the layer looked sufficiently uniform and quite 

 free from abrasion of the silk. 



The mean thickness of the double silk covered wire used in the last 

 winding was 0-01914 inch, so that the outside diameter of the wound 

 coil, calculated from the value given above for the coil wound with bare 

 wire, was 



21-04488 inches at 20°-4C. 



The coil was then brushed over with melted paraffin wax, bound round 

 with silk ribbon that had been soaked in a solution of shellac, and finally 

 loosely covered up with a wide silk ribbon that had been passed through 

 paraffin wax. 



