LAWS. — COEFFICIENTS OF SELF-INDUCTION. 271 



a minimum of sound in the telephone; then adjust for silence by vary- 

 ing the field current. Read the micrometer head and value of exciting 

 current. Repeat this process at the terminals of the unknown coil. 

 These measurements furnish the data necessary for the calculation of 

 L and R. 6 is given by difference of micrometer readings divided 

 by 20, as before. Owing to lack of exact similarity of E. M. F. waves 

 it would be necessary in practice to adjust by a minimum, annulling 

 the fundamental. 





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OQ — * 



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V "'V<*» 



J 



Fig. 7. 



(c) The trouble of taking the preliminary curve, as well as any 

 uncertainty which its use may introduce, can be avoided by the con- 

 nections shown in Figure 7. 



This method differs from the preceding only in the use of a derived 

 E. M. F. for balancing, and in the method of its adjustment. For con- 

 venience give R t such a value that the P. D.'s around Rx and R shall 

 be about equal. Adjust the phase of Armature A by the tangent 

 screw, and adjust for amplitude by varying the field current, final 

 adjustment being obtained by the slider. 



Now change the connection to the terminals of the unknown coil, 

 and again adjust for silence, making the adjustment for amplitude in 

 this case by the slider alone. If the two values of the slider resist- 



T 7? 



ance are R* and R 3 , then — = ^ . 6 is given by the micrometer 



Jb r±r% 



head as before. 



Fig. 8. 



(c?) "We may use a differential telephone, that is, a telephone hav- 

 ing two opposed windings of equal influence and resistance, in the 



manner shown in Figure 8. 



