1876.] 



Determination of Verdefs Constant, 



145 



Tielix without an iron core, and bisulphide of carbon, enclosed in a tube 

 with glass ends placed within the helix, was chosen as the medium. 



The strength of the current in the helix was deduced from the deflec- 

 tion of a small magnet suspended near to it and outside it, and the 

 rotation was measured by means of a divided circle. 



The investigation then resolved itself into three parts : — 



1. The determination of the constants of the helix. 



2. The determination of the ratio which the rotation per unit of length 

 bore to the tangent of the deflection of the suspended needle. 



3. The determination of the horizontal component of the earth's mag- 

 netism at the time and place of observation. 



The Experiments. 



Determination of number of luindings. 



To determine the number of windings, it is necessary to know the dif- 

 ference of magnetic potential at the ends of the helix when a unit current 

 passes in the wire. 



To determine this the author places the helix and great dynamometer 

 coaxial, and suspends a magnet and mirror at the centre of the dyna- 

 mometer. By sliding the helix endways along the axis, so as to bring 

 different points of it over the suspended mirror, he obtains the magnetic 

 intensity at these points in terms of that of the dynamometer, which is 

 known. Varying currents are set in opposite directions through helix 

 and dynamometer till the action on the suspended magnet is zero. By 

 integrating these values along the axis between limits corresponding to 

 the ends of the helix the difference of magnetic potential at the ends for 

 a unit current is determined. A rule known as "Weddle's (see Boole's 

 ' Finite Differences,' p. 47) is used for the integration. 



This difference is called N, and from it is deduced the number of 

 windings {n) by Maxwell's 'Electricity,' art. 676. After describing the 

 mechanical arrangements and giving a drawiug of the connexions, the 

 author gives a Table showing the results of the experiments for the 

 determination of N. 



The final results are 



N= 10752, 

 n = 1028-15. 



By an equation of units N is shown to be the ratio of two things of the 

 same dimensions, and therefore a number. 



Determination of Areas. 



To calculate the strength of a current in a helix from the deflection 

 of a magnet suspended outside it, it is necessary to know 2(A), the sum 

 of the areas of the windings. 



This was obtained by comparing the action of the helix on such a magnet 



