446 



ELECTRICAL MEASUREMENTS 



is vertical and includes the point 0, the rotating field will cut the 

 cylinder, induced currents will be set up and the cylinder will 

 rotate just as does the disc in the inverse of Arago's experiment. 

 This was one of Ferraris' classic experiments. Ferraris, however, 

 did not appreciate the importance his discoveries would assume 

 when developed along engineering lines, for he states that 

 "These calculations and experimental results confirm the evident 

 a priori conclusion that an apparatus founded on this principle 

 cannot be of any commercial importance as a motor, and while 

 we may study the dimensions in order to increase notably its 

 power and output, it would be useless here to enter upon any 

 consideration of this problem. Still, the experiments described 

 may be of some interest." 1 



The possible application' of the principle to measuring instru- 

 ments was, however, mentioned by him. 



Application to Measuring Instruments. The strength of the 

 field and therefore the action on the cylinder will be greatly 



FIG. 258. Magnetic circuit and rotor of an induction instrument. 



increased if the coils be provided with laminated iron cores. 

 Fig. 258 shows the magnetic circuit of an induction-type, watt- 

 hour meter made at one time by Siemens and Halske. 



In the figure, A A is a laminated ring with four poles FF, EE 

 projecting toward the center and C is a circular laminated iron 

 core. It is evident that the poles F and F produce a field whose 

 general direction in the right-hand diagram is horizontal, while the 

 field due to E and E is vertical. The strength of field in the nar- 

 row air gap will be considerable and in this field is placed the 



