:'. I Till-: DIRKCT-CURRKNT MoToli < II. II 



and greater than E, ?t is negative, i.e. the direction of 

 rotation is reversed, and consequently the induced tension 

 is of the same sign as the tension of the line. Suppose 

 that the load is such that, to balance it, a current Dh '" is 

 required. For uniform motion we must have 1L" K" the 

 tension, of the line, plus K" F" the induced tension, equal 

 to H" F", the heat drop. We thus have a greater current 



flowing than that given by c= . 



h 



It is important to notice that the direction of rotation 



Tjl 



is reversed only when the current is greater than - ; this 



R 



shows that we can reverse the motion of a dynamo by 

 putting on a load greater than that given by 1*41 = ; 



/I 



the dynamo then ceases to be a motor and becomes a 

 generator. 



The conditions here described can be represented 

 experimentally as follows : A motor is taken and con- 

 nected up to a line of constant tension through an 

 ordinary starting rheostat, and an adjustable rheostat is 

 placed in series with the armature, so that the resistance 

 can be varied and the motor brought to a standstill. 

 On the shaft is placed a pulley with a cord attached, 

 which is wound round the pulley and hangs down on one 

 side, loaded with weights in a pan. The weights and the 

 rheostat are adjusted until the armature is at rest, and on 

 turning it with the hand, as ready to move one way as the 

 other, the current through the armature being observed 

 with an ammeter. 



The following experiments may be made : 



1. Increase the load by adding weights and observe 



