.)(> 



cent, of the terminal electromotive force. 

 The length of the line OR in the diagram 

 will therefore be, at most, one -twentieth 

 part of OE. When the self-induction is 

 negligible, the point A will almost con- 

 cide with O, and E' will almost coincide 

 with E", which shows that the safe mar- 

 gin between minimum and maximum 

 field-strength, and, consequently, also 

 between the smallest and largest per- 

 missible exciting current, is extremely 

 narrow. Machines of this type can only 

 be run in parallel if the strength of their 

 fields is adjusted with almost mathe- 

 matical precision, and as this would 

 require more skill and attention than is 

 available with the ordinary staff of a cen- 

 tral station, such machines are practically 

 unfit for parallel working. To make 

 them fit for this method of working 

 either the armature resistance or the self- 

 induction must be increased. An in- 

 crease of resistance, in order to be 

 effective, would have to be so consider- 

 able (as is easily seen from the diagram), 

 as to seriously prejudice the electrical 



