10 THE DIHECT-CUKRENT MOTOE CH. I 



the machine is at rest, shows that the force produced by a 

 current in the armature is independent of the motion of 

 the armature. 



In Fig. 4 will be found the results of a torque test 

 made with an Edison 12 K.W. dynamo. This machine 

 has 200 conductors on the armature, and is designed to 

 carry 96 amperes at 125 volts, running at 1,500 revolu- 

 tions per minute. The conductors are laid on the surface 

 of the iron core. The torque is plotted 011 a base of 

 amperes in the armature. 



The current in the magnets was kept constant at 4 - 5 

 amperes. The leverage was 30 inches. A pull of 1'4 pounds 

 due to friction was observed with no current in magnets or 

 armature ; this was not increased when the magnets were ex- 

 cited, showing that the hysteresis torque was inappreciable. 

 The induction factor is obtained from Equation 5. 



After the experiment a current of 78 amperes was 

 passed through the armature, and a fall of potential of 

 3 '74 volts observed, giving the resistance from brush to 

 brush as 0-048 ohm. 



In Fig. 5 let ab represent the length ~jrr, where r is 

 the mean distance of the axes of the conductors from 

 the centre of the shaft ; ab is thus equal to one half of 

 the armature circumference measured round the circle of 

 radius r. 



Set off along ab points 1, 2, 3, 4 ... to represent the 

 positions of the conductors. The distance from point to 



point will be . of the complete circumference, assuming 



for the present that the conductors are laid side by side. 

 At each point draw a vertical line, to represent by its 

 length the number of lines of force per square centimetre 



