INDUCTANCE AND CAPACITY 



343 



dividing the primary into two equal sections, properly separating 

 them, and placing the secondary midway between them, all three 

 coils being coaxial, a satisfactory arrangement may be obtained. 

 If the diameter of the secondary coil is such that the mutual 

 inductance is a maximum, the secondary will be so placed that 

 its circumference is in a zero field. For this reason a small varia- 

 tion in the diameter or a small axial displacement of the coil 

 will produce only a slight variation in the mutual inductance. 

 The construction is indicated in Fig. 203, where the proper rela- 

 tive dimensions are shown. 



With the proportions indicated a multiple layer secondary 

 having a considerable cross-section (0.5 cm. by 0.5 cm.) may be 

 used. The effect of variations of the diameter of the secondary 

 coil are shown below. The mutual inductance is a maximum 

 when A = 14.58 cm. If the secondary circuit is displaced from 

 the midposition between the primary coils by 0.35 cm., ra is 

 reduced by less than 1 part in 10,000. 



CAMPBELL STANDARD OF MUTUAL INDUCTANCE 

 a = 10 cm., b = 7.5 cm., nin 2 = 100,000 



Variable Mutual Inductances. Variable mutual inductances, 

 in other words, air core transformers of variable ratio, are ex- 

 tremely useful in alternating-current measurements, as, for 

 example, in determining self-inductances and in measuring the 

 ratios of instrument transformers. 



For the highest utility the coils should be wound astatically, 

 expecially if the apparatus is to be used in an electrical engineer- 

 ing laboratory, and even then one should satisfy himself by tests 

 that stray field effects, due to non-uniform fields, are absent. 

 If an astatic arrangement is not used, great care must be taken 

 that the standard is not set up where there are alternating stray 

 fields or where its field will influence other instruments. 



It is desirable that the scales of variable mutual inductances 

 be as uniform as possible. By the use of Lord Rayleigh's ar- 



