490 



PROFESSOR W. E. AYRTON, MR. T. MATHER AND MR. F. E. SMITH: 



TABLK IV. Results of Measurements of the Diameters of the Coils, to Centres of 

 Wires, on Suspended Cylinder No. 2. Temperature 15'5 C. 



The following convention is adopted in numbering the 14 axial planes. The upper 

 plane end of a cylinder is viewed and one of the two marked diameters nearest in line 

 with the connectors of the coils is called No. 1. The direction of ascending numbers is 

 clockwise, diameters 1 and 14 being on opposite sides of the plane containing the leads. 



An idea of the probable error of the mean diameter of any one coil may readily be 

 obtained. The values of the standards of lengths employed are known in terms of 

 the National Physical Laboratory 12-inch end gauge of similar type standardised by 

 the Board of Trade. The absolute values are not of importance, however, for if the 

 dimensions of the fixed and suspended systems change in the same proportion and in 

 the same direction, the force due to the current is unchanged. For the smaller coils, 

 an 8-inch steel gauge was used ; for the larger ones, this was combined with a 5-iuch 

 gauge. The ratio of the lengths of these gauges was known with an error certainly 

 less than 5 in 1,000,000. The probable error due to the setting of the gauges in the 



