7 6 PROPERTIES OF CIRCULAR CURRENTS. 



On the other hand, the value of the mean radius a is 



which gives 



2r 7 \/ 



Substituting in equation (7), we get 



/ o\ 2 9 i 



(8) tfj=fl2 + 



Calling /* the distance ~ of two successive windings, we shall 

 2 



give to a the successive values o^h^zh ..... (/- i)/i; it follows that 



2-3 



and therefore 



i 

 i -- > 



4 \ P 



i 'Vi i 



3 \ //\ 2/ 



As the thickness 2^ of the channel is equal to ph, we get from 

 this 



(9) -+- 



The radius a^ of the ;^ circle differs thus appreciably from the 

 radius a of the mean circumference. 



If the windings are so close that the last term of correction may 

 be neglected, the radius a only depends on the dimensions of the 

 channel. 



