6 THE DESIGN OF STATIC TRANSFORMERS 



intervals and record their " constants." One day we wei v 

 accused of having given out the constant of one of these 

 wattmeters as 10 per cent, too low ; for a certain standard 

 transformer, which was kept permanently in the testing room, 

 was remeasured, and the readings indicated a core loss some 

 10 per cent, less than its known value as determined by many 

 previous measurements. We investigated the instrument 

 carefully, but could only confirm our original calibration. 

 During two or three days we did little else than overhaul 

 standards and check instruments against one another. Then 

 it was definitely found that the same wattmeter at certain 

 times indicated the core loss of a given transformer to be 

 10 per cent, greater than at other times. Further investiga- 

 tions disclosed the fact that the low readings were obtained 

 when the circuits were supplied from an alternator with its 

 armature windings embedded in slots, while the high readings 

 were obtained when an alternator was used whose armature 

 windings consisted of flat coils resembling " pancakes " and 

 bound down on the surface of the armature. Thus the watt- 

 meter was vindicated, and we were impressed with the 

 practical significance of the shape of emf wave supplied by 

 an alternator. The slotted alternator was one of the earliest 

 which our firm had built. It was, of course, a single-phase 

 machine, and it had only one slot per pole. This gave it a 

 very "peaked" wave. We confirmed this by determining by 

 the Joubert contact method (using our Thomson quadrant 

 electrometer) the wave shapes of both machines not only on 

 no load, but also on full load. The old "pancake" or 

 " smooth-core " type had practically a sine wave both at no 

 load and at full load, while the wave of the iron-clad alter- 

 nator was very peaked under both conditions, and (for a given 

 terminal pressure) had a crest pressure some 10 per cent, 

 higher than the crest pressure of the " smooth-core " alter- 

 nator. In other words, the form factor (defined by Fleming 

 on p. 583 of Vol, I, of the 2nd edition of his " Alternate^current 



