L82 Mr. 0. Heaviside on the Resistance 



with yr for the electromagnet, we may neglect it; and then (5) 

 becomes 



Ri . A. .L" 

 1+m IP 



V 



or R! = Lm approximately. The resistance of the electro- 

 magnet for high speeds varies inversely as the speed to obtain 

 the maximum strength of signals. 



Using the value of =5 given in (9), we have 

 R T 



E i 14-64 ?=2.s» 



x+y 



where T= — 

 m 



At 100 words per minute Morse code, T = about -£§ second; 



therefore at this speed 



Suppose ,v = 2, y=z=l centimetre; then 



^=195-2; 



or the resistance of the electromagnet is ytr 5 th part of the ex- 

 ternal resistance to obtain the maximum magnetizing force — 



a very low result. If ^~ is taken into account, this becomes 



T ^th. 



7. Having made the magnetizing force a maximum for a 

 given speed and size of electromagnet, by varying the thick- 

 ness of the wire, we may next find the ratio between the outer 

 and inner radius of the coil to make the attractive force be- 

 tween the core and a soft-iron armature a maximum. We 

 have 



r= E 



^(R + R^ + L 2 ™ 2 

 (neglecting L^), where 



L = § 7r 2 ln%v-yy 2 (jj 2 + 2ay + 3y 2 + 24>xkz 2 ) , 

 R = Trp In* ( or — y 2 ) . 



