OPERATION OF THE DYNAMO AS A GENERATOR. 91 



man silver ribbon, connected across the terminals of its series 

 field winding. By adjusting the resistance of this shunt once 

 for all, any desired fractional part of the current output of the 

 machine may be made to flow through the series field winding. 

 The machine is manufactured with a greater number of turns of 

 wire in its series field winding than are strictly necessary, and by 

 changing the resistance of the above-mentioned shunt, the machine 

 may be adjusted in the factory for flat-compounding, or for any 

 desired degree of over-compounding. This adjustment should 

 be made when the machine is at its normal running temperature. 

 Fig. 69 shows a compound generator (short-shunt) with the above- 

 mentioned shunt, S, between the terminals of the series field 

 winding. 



Over-compound generators are used where current is to be de- 

 livered to lamps or motors over a long transmission line and where 

 it is desired to maintain an approximately constant voltage at the 

 terminals of the lamps or motors in spite of the increased loss of 

 electromotive force in the line (line drop) with increase of current. 

 For example, railway generators are usually over-compounded so 

 as to give 500 volts at the generator terminals when the current 

 output is zero, apd 550 volts at the generator terminals when 

 the current output is at the normal full load value. In this 

 case the machine is said to be 10 per cent, over-compounded, 

 inasmuch as the full load voltage is 1 10 per cent, of the no load 

 voltage. 



51. Brush lead. Fig. 71 represents a two-pole generator 

 with ring armature. The fine curved lines show the trend of the 

 magnetic flux from the north pole of the field magnet into the 

 armature core, and from the armature core into the south pole of 

 the field magnet. 



The turns of wire as they pass the points a and 3, Fig. 71, 

 have zero electromotive force induced in them, whereas the elec- 

 tromotive force induced in the individual turns of wire becomes 

 greater and greater on either side of the points a and b up to the 

 pole tips, and approximately equal electromotive forces are 



