534 MEASUREMENT OF RESISTANCES IN ABSOLUTE VALUE. 



The following table has been calculated by Lord Rayleigh :' 



In order to diminish the error made in the distance x of the 

 mean planes, the coils are successively turned about, so that in 

 the experiments they occupy all the relative positions. This has 

 led observers to use coils with cores of bronze, turned with care 

 on the outside. 



M. Mascart used unequal coils, placed concentrically, with their 

 mean planes in coincidence. The calculation of M is then made 

 by formula (18) of 762. 



M. Roitif has proposed to use a closed solenoid as induced coil. 

 The coefficient of mutual induction between a circular solenoid, 

 containing n l windings for unit of arc, and a wire coiled ri times 

 round the first, is expressed by 



JM if n it i n, | , 



X 



x being the radius of the circumference on which is the element 

 d of the section. 



If the section of the ring is a circle of radius a y and if R is 

 the mean radius of the ring, we have (502) 



if it is a rectangle of sides 2a and 



These expressions are very simple, but the regular coiling of 

 the wire would prevent very great difficulties. 



* Lord RAYLEIGH. Comparison of Methods for the Determ. of Resist. 1884. 

 f ROITI. Atti deWAc. di Torino. April 3Oth, 1882. Nuovo Cimento [3], 

 Vol. xv., p. 97. 1884. 



