Alternating Current and Electrodynamometer. 143 



Let R' be the resistance necessary to balance the electro- 

 dynamometer, 

 L the known self-inductance added, 

 R" the resistance, to produce the new balance, 

 Rj the increase in resistance, 

 R the resistance corresponding to -01 henry, 

 R" x and R x the values for the resistances obtained with X in 

 series. 



R' 



L 



R" 



Ri 



R 



R x " 



R x 



X 



180-8 



•02 



212-1 



31-3 



15-65 



198-45 



17-7 



•0113 



179-97 



•035 



234-92 



54-95 



15-7 



197*62 



17*65 



•01121 



Experiments made by students using other methods (see 

 Carhart and Patterson's measurements) gave for the same coil 

 values lying between -0107 and '0117 henry as the extreme 

 limits, but mostly nearer to the average, which exactly agrees 

 with the second value given above. 



The capacity and the self-inductance of the ordinary resist- 

 ance coils exclude a still greater accuracy, but I feel sure that 

 this can be obtained by employing resistance free from those 

 faults. A preliminary experiment with a fine wire gave as the 

 length that had to be added, when the self-inductance of the 

 standard was increased from *005 to '015, from *015 to -025 and 

 from *025 to *035 henry, 810, 812, 814 divisions respectively. 



I believe this method to be of some advantage, when we 

 have an electrodynamometer of large self-inductance. The 

 possible errors are small, since only one resistance-box is used 

 for the adjustment, and the experiment can easily be performed 

 by even inexperienced students. 



Physical Laboratory of the University of Michigan. 



