94 HENRY A. ROWLAND 



" = J -/ y * ~ A ' ( e r (-*> rx> ) (5^) 



ZVTU? A'? b 1 v 



which gives the number of lines of induction passing out from the rod 

 along the length AL when the helix is symmetrically placed on the rod. 

 To get the number of lines of induction passing along the rod at a 

 given point, we have 



f\Z (L 1 A I 



where 



c rt 1 



When the bar extends a distance L' out of both ends of the helix, so 

 that 



if = */RW and A' = 



we have 



It may be well, before proceeding, to define what is meant by mag- 

 netic resistance, and the units in which it is measured. If ft is the 

 magnetic permeability of the rod, we can get an idea of the meaning 

 of magnetic resistance in the following manner. Suppose we have a 

 rod infinitely long placed in a magnetic field of intensity parallel to 

 the lines of force. Let Q' be the number of lines of inductive force 

 passing through the rod, or the surface-integral of the magnetic induc- 

 tion across its section; also let a be the area of the rod. Then by 



definition n = -sL. If L is the length of the rod, the difference of 



flEty 



potential at the ends will be LS& ; hence 



0' - L and fl - - L - L 



^ X ' ~ IT ~^' 



and R in the formula? becomes 



R _ R, _ . 1 



-ft -jL . 



L* a/j. 



It is almost impossible to estimate R' theoretically, seeing that it 

 will vary with the circumstances. We can get some idea of its nature, 

 however, by considering that the principal part of it is due to the 

 cylindric envelope of medium immediately surrounding the rod. The 

 resistance of such an envelope per unit of length of rod is 



