53 



to about half their maximum output or 

 over, but that machines working under a 

 less load cannot, with certainty, be so 

 coupled. A glance at the diagram, Fig. 

 10, shows the reason for this. The 

 margin of field strength is represented 

 by the difference in the length of the 

 lines OE' and OE. This difference de- 

 pends, to a great extent, upon the length 

 of the line OA, which is proportional to 

 the current. If the current through the 

 machines already at work is small, it will 

 be further reduced as soon as the new 

 machine is switched on, and under these 

 conditions the margin of field strength 

 may become so small, that a slight 

 reduction of the exciting current of one 

 machine may cause this to be over- 

 powered by the other machines, and 

 hence the experiment may fail. If, how- 

 ever, the current through all the machines 

 is sufficiently great, then the margin will 

 also be sufficient to cover such irregu- 

 larities in the exciting currents, and in 

 the torque of the engines, as may be 

 expected to exist when the plant at a 



