106 ELECTRICAL MEASUREMENTS 



primary. The arrangement, commonly called a solenoidal 

 inductor, is indicated in Fig. 54. 



Exploring 

 Coil 



Rh. Allu 



Sec. 



FIG. 54. Solenoidal inductor. 



Let I be the length of the primary coil, a its radius, n P the 

 number of primary turns per unit length, Ns the total number 

 of secondary turns and A the cross-section of the primary. Then 

 if I is many times a the mutual inductance is approximately 



m 4-jrnpNsA 



and 



8wn P N s AI 

 ^ = r 



all quantities being in the c.g.s. system, 

 or 



Q, I and r in the practical system. 



Ordinarily when this method of calibration is employed, the 

 instrument is to be used in a closed circuit. The damping will 

 therefore be dependent on r and the constant of the galvanometer 

 becomes a function of the resistance of the circuit. For this 

 reason, it is common in magnetic work to arrange the apparatus 

 so that the secondary of the calibrating solenoid is continuously 

 kept in circuit with the galvanometer and the exploring coil. A 

 substitution method is customarily used. First, a known change 

 of flux is produced in the circuit by the mutual inductance and 

 the change of flux per scale division of the galvanometer thus 

 determined. After this, an observation of 0/, corresponding to 

 any change of flux in the specimen, enables the change in linkages 

 to be calculated. If r is altered, recalibration is necessary. 



