MAGNETIC EFFECT OF ELECTRIC CURRENT. 103 



shown by the fine lines in Fig. 67, and the intensity of this field 

 is given by the equation 



H-firsI (34) 



in which H is the intensity of the field in gausses, z is the num- 

 ber of turns of wire on each centimeter of length of the solenoid, 

 and / is the strength of the current in abamperes. If the current 

 is expressed in amperes equation (34) becomes 



H -- 



IO 



(35) 



in which H, as before, is expressed in gausses. 



In order to derive equation (34) let us consider the arrange- 

 ment shown in Fig. 68 consisting of a long coil having z turns 



long coil 



GGS00 



!%%g0g$0flKrn?^^ 



side view (section) 



Fig. 68. 



long coil 



end view 



of wire on each centimeter of its length, with a steel rod pro- 

 jecting into it. Let us assume that the total pole strength m on the 

 end of the steel rod is spread uniformly * over a portion of the rod 

 of length /, as indicated by the shading in Fig. 68. The lines of 

 force emanate from such a long pole in planes at right angles to 

 its length as shown in Fig. 68, and the intensity of this field at 

 the surface of the long coil is H 1 = zmfrl, according to Art. 40, 

 where r is the radius of the solenoid and / is the length in centi- 



* It would be very difficult indeed to magnetize a rod so as to have its pole spread 

 uniformly over a given length of the end of the rod, especially when the rod projects 

 into a solenoid as shown in Fig. 68, because the effect of the solenoid is to tend to 

 concentrate the magnet pole at the end of the rod. The assumed distribution of pole 

 is, however, a possibility if the current in the solenoid is very weak and therefore the 

 assumed distribution is a legitimate basis for the discussion of equation (34). 



