1895.] on the Absolute Measurement of Electrical Besistance, 619 



July 14th, afternoon. — Standard coil readjusted. Resistance coils 

 removed and replaced. Three-minute tapes. 



0-999853 

 0-999866 

 0-999875 



Mean 



0-999865 



These results show that no single observation differs from the 

 mean of a number by more than one part in ten thousand. 



We have now to consider the other factor in the resistance — the 

 coefficient of mutual induction of the coil and disc. This depends 

 merely on the dimensions of the coil and disc if they are accurately 

 centred, and as the adjustment for centre was made in between the 

 successive sets given in the last table, we may take for granted that 

 it can be made with the requisite accuracy. 



The circumference of the disc is a sufficiently true circle, the disc 

 having been ground true in place. Disc and axle were then removed 

 and transferred to my Whit worth measuring machine. The measure- 

 ment of the disc's diameter presented no difficulty. It was easily 

 determined to the ten-thousandth of an inch. The mean radius of 

 my coil cannot be determined with the same accuracy, but I believe 

 it is known to the thousandth of an inch. The coil consists of a 

 single layer of silk-covered wire wound in a screw thread cut in a 

 brass frame. The silk covering of the wire introduces some un- 

 certainty. It was measured bolted to its stand as in use. Measure- 

 ments were taken along 18 diameters in the Whitworth machine with 

 the following results : — 



These measures clearly show that the coil is elliptical in section, 

 the difference between the major and minor axes being about • 008 in., 

 or about one part in 1300. The way in which this fact emerges from 



