36 DIRECT CURRENT GENERATORS. [Exp. 



by the use of a ballistic galvanometer (or voltmeter or ammeter used 

 as a ballistic galvanometer) ; the chief advantage of the method is 

 that it does not require facilities for running the machine. With 

 the armature stationary and the galvanometer connected to the ter- 

 minals of one field, break a certain armature current and note the 

 throw of the galvanometer. Repeat, breaking the same armature 

 current with the galvanometer connected to the other field. The 

 ratio of galvanometer throws gives the desired ratio of field turns. 

 It is best to take a series of readings and average the results. 



31. An estimate of the number of turns in a coil can be made 

 from its measured resistance, size of wire and mean length of turn. 

 This can be used as a check, but the method is commonly only 

 approximate on account of the uncertainty of the data. 



32. To Compound a Generator by Testing with Added Turns. 

 The proper number of series turns required to compound a generator 

 can be ascertained by trial by means of temporary auxiliary turns. 

 With the generator running at full load" pass current from an inde- 

 pendent source through these auxiliary turns and adjust this current 

 until the terminal voltage of the generator has the desired full-load 

 voltage. This current (say 220 amperes), multiplied by the number 

 of auxiliary turns (say 10) through which it flows, shows that 2,200 

 extra ampere-turns are needed at full load. If the full-load current 

 is 200 amperes, the generator would accordingly require n series 

 turns. . 



33. Full-load Saturation Curve. For obtaining this curve, the 

 field excitation is varied and the load adjusted at each reading, so 

 that the external current remains constant at its full-load value. 

 Field currents are plotted as abscissae and terminal voltages as ordi- 

 nates. Such a curve is to be taken later (Exp. 3-A) on an alter- 

 nator; it may accordingly be omitted, in the present experiment, if 

 time is limited. 



