METHODS OF TESTING ALTERNATORS 281 



determination of L made by this method in the laboratory of the 

 Societe Sautter-Harle & Company of Paris, which employs with success 

 the methods described in this note. The curves of the first three 

 figures have been obtained by means of the author's oscillograph at 

 the terminals of an alternator of 350 K.W., and represent the periodic 

 curves of the difference of potential between these terminals when 

 the alternator works on open circuit, then when delivering 62 

 amperes, and finally 102 amperes, its normal load. The effective 

 value of the e.m.f. was 1155 volts in star. The vertical lines 

 represent the tracings produced by the contact of the contact-maker 

 S. Figs. 130 and 150 are printed upon one and the same sheet of 

 paper, by causing the vertical lines to coincide, from which Fig. 16 

 is produced, which shows clearly in evidence the dephasing between 

 the internal e.m.f. E, and the pressure at terminals U. The 

 dephasing reaches 24 30' in the test with 102 amperes, and 19 in 

 the test with 62 amperes. In applying the preceding formula (with 

 r=2.5 ohms) the values are obtained: 



oZ,= (2.5X62-fii55) Xo. 344= 7. 26 ohms, 



566=7.8i ohms, 



which differ little (and would perhaps be equal if greater precision 

 had been taken in the measurement of the phase-difference between the 

 two curves). In practice one would take the mean value 7.5 ohms. 



It is evident that it is easy thus to determine the constant of 

 distortion, and this justifies the employment of the theory of the two 

 reactions rather than the rough method of the " curve of short-circuit," 

 which combines together the two often very different reactions. 



The ondograph gives the same result, if one marks off' successively 

 upon the same sheet of paper the e.m.f. upon open circuit and then 

 that on closed circuit, and each time the mark obtained when the 

 apparatus is traversed by the current coming from the brushes BB' . 

 It is possible to dispense with making this mark, if the ondograph 

 is. driven, not by a synchronous motor, but by a flexible coupling 

 connected mechanically to the shaft of the alternator (with the inter- 

 position of gear wheels, as in the recording mechanism of Francke). 



Moreover, even in the absence of the preceding analyzing appa- 

 ratus, one can approximately obtain the transverse reaction, or rather 

 its ratio to the direct reaction, with unsaturated field-magnets, by 

 sending, as M. Herdt has already suggested, into the alternator arma- 



