280 METHODS OF CALCULATION 



from which 



(uL= I r+-r } tan </>. 



With the oscillograph, for example, it suffices to place upon the 

 shaft, or upon one of the pole pieces of the rotating field-magnet 

 (Fig. 60), a contact segment S upon which rub two brushes BB', 

 connected in series with a battery, one of the oscillographs O of a 

 double-oscillograph apparatus, and the electromagnet e which controls 

 the release of the shutter. The contact segment S, insulated from the 

 base, closes the circuit upon its passage below the brushes, and opens 

 it again at the precise moment when the brush B leaves it. A segment 

 is selected large enough in order that the shutter shall have time to 

 open before the rupture of the circuit, in order that the latter may then 

 be photographed upon the plate in the form of a vertical line inter- 

 secting the straight-line zero furnished by one of the oscillographs. 



FIG. 28. 



The second oscillograph of the system, O', is connected to the terminals 

 of the alternator and serves to register the e.m.f. U at the terminals. 

 Two experiments are made on two different plates, or upon the same 

 plate after having slightly displaced the zero-line produced by the 

 oscillograph O, so as to distinguish the two different records. The 

 e.m.f. at open circuit is then marked upon it, and next the e.m.f. 

 when the circuit is closed upon a dead resistance, at the same time 

 that the current 7 delivered thereto is measured. In these two tests 

 the distance is measured from the zero of the curve of e.m.f. to the 

 point of intersection by the vertical with the zero-line. The difference 

 between these two lengths thus measured determines the displacement of 

 phase of the e.m.f. at the terminals, that is to say, the angle of dephasing 

 sought (taking for the value of 271 the length of a period measured upon 

 the plate, and taking the ratio of the measured retardation to this 

 length). It is sufficient to take the values of <p, of 7, and of U in the 

 preceding formula in order to determine the transverse reaction L. 

 Figs. 130, 140, isa and 16 (Chapter I) represent an example of the 



