Induction Motor. 161 



The resistance R of each of the circuits formed by joining' a pair 

 'of strips in series by two perfect conductors will be 



R = 2/=4«cr 

 But R=:?t/) ;uid .'. /3=4a- 



That is, if the ends of the conducting strips were joined by per- 

 fect conductors, we would have p equal to four times the resist- 

 ance of the copper from end to end. The remaining factor of five 

 is due to tlie fact that the effective resistance of the paths joining 

 the strijis is not zero. 



The large increase in the value of A. in the experiments with the 

 ■Cradle dynamometer is probably due to the fact that in these 

 ..experiments the air gap between the copper and the pole pieces was 

 .smaller than in the case of the permanent magnet. 



Another point, which must be determined, before it is possible to 

 ■design a new apparatus, is the* extent to which the magnetic resist- 

 ance of a magnetic circuit can be inferred from the dimensions of 

 'the apparatus. 



To do this, the " Immisch " motor was used, and the flux thread- 

 ing the iron core Avas measuied for different diameters of the core, 

 .and different magneto-motive forces, with the following results : — 



The numbers in the last column give the effective magnetic resist- 

 :ance of the Circuit. 



Comparing these values with the values of the resistance as cal- 

 culated from the dimensions, it will be noticed that for small air 

 gaps, the effective resistance is larger than the calculated, and also 

 that the value increases with increasing flux density. This means 

 a higher magnetic leakage at higher flux densities. But Avhen the 

 .air gap is 2. cm. long, the calculated and effective magnetic resist- 

 ances are practically the same, and also the value of the effective 

 a-esistance is almost independent of the flux density. 



Certain dimensions in the new apparatus are fixed : — 



(i.) The external diameter of the rotor is limited to 30 cm. 

 (ii.) The internal diameter of the pole pieces must he not less 



than 7.3 cm. 

 .(iii.) The external diameter of the armature must not exceed 

 5.5 cm. 



