596 A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



where / is the length of the wire, and p. is the coefficient of magnetic induc- 

 tion for the substance of the wire. 



(116) The dimensions of the coil used by the Committee of the British 

 Association in their experiments at King's College in 1864 were as follows: 



metro. 



Mean radius =a= '158194 



Depth of each coil =& = -oi608 



Breadth of each coil = c = '01841 



Distance between the coils ='02010 



Number of windings = 313 



Diameter of wire ='00126 



The value of L derived from the first term of the expression is 437440 

 metres. 



The correction depending on the radius not being infinitely great compared 

 with the section of the coil as found from the second term is 7345 metres. 



The correction depending on the diameter of the wire is 1 



e , - + '44997 



per unit ot length J 



Correction of eight neighbouring wires + '0236 



For sixteen wires next to these + -0008 



Correction for variation of current in different parts of section '2500 



Total correction per unit of length '22437 



Length 31T236 metres. 



Sum of corrections of this kind 70 



Final value of L by calculation 430165 ,, 



This value of L was employed in reducing the observations, according to 

 the method explained in the Report of the Committee*. The correction de- 

 pending on L varies as the square of the velocity. The results of sixteen 

 experiments to which this correction had been applied, and in which the velocity 

 varied from 100 revolutions in seventeen seconds to 100 in seventy-seven seconds, 



* British Association Reports, 18G3, p. 169. 



