1881.] Determination of the Ohm in Absolute Measure. 127 



one revolution shifted the position of rest through 5*6 scale divisions, 

 corresponding to an angle of "001107. 



Another experiment in which the magnet was turned in the opposite 

 direction gave '001117. 



Hence t= ' Q *p 1 =-00018. 



The correction due to torsion is best applied to the value of G at 

 the same time as the correction for level by writing everywhere — 



G l±fi*Bl for G 

 1 + t 



Constants of the Coil. — The accurate determination of the constants 

 of the coil forms the most difficult part of the measurements. Un- 

 winding the coil, the outer circumference of the successive layers was 

 measured by means of a steel tape. Each measurement was repeated 

 several times, and the agreement was satisfactory. The thickness of 

 the wire was found to be 1'37 millims., which, on the circumference of 

 the successive layers, should produce a constant difference of 2*74 ir or 

 8" 62 millims. Owing, however, to defective winding, each layer enters 

 a little into the grooves of the subjacent layer, and the differences in 

 circumference of successive layers were therefore always smaller than 

 they ought to have been. The differences varied between 7' 7 millims. 

 and 8" 6 millims., but generally were about 8"1 millims. The wire was 

 a little too thick, and as it had been taken off. the coil and rewound, 

 small irregularities were formed which prevented a satisfactory wind- 

 ing. Each coil had 156*5 windings. Of these 156 were in one coil 

 regularly distributed over twelve layers of thirteen windings each ; 

 while half a turn was outside. In the second coil the twelve layers 

 only contained 155 windings, and one turn and a half was lying 

 outside. In the calculation for mean radius it was assumed that each 

 complete layer contained the same number of turns. Let S be the 

 sum of all measurement for one coil, also C the circumference of the 

 layer containing the loose extra turns ; then we find the mean circum- 

 ference of the first coil, = 99 -680 



15(r5 



and for the second, fl3-l/12)S + l-5 C = Q9 



156'5 



Or as the circumference of the outside of the mean turn . = 99 '666 

 From this is to be subtracted a correction equal ir x thick- 

 ness of tape = "031 



99-635 



