Effect of a Changing Magnetic Field. 



473 



At first it Avas thought that the value of the restoring 

 couple could be deduced from a knowledge of the moment of 

 inertia of the swinging body and its obsened period. Since, 

 however, the restoring couple is due almost entirely to the 

 electrostatic field the motion could not be simple harmonic, 

 and the period therefore bears no known simple relation to 

 the directive moment. The plan finally adopted was to 

 determine the moment of torsion of the upper suspension 

 wire by observing the period with a body of known moment 

 of inertia suspended, and then making series of derlexions 

 due to the electromagnetic effect alternately with similar 

 series due to turning tlie torsion-head back and forth through 

 a known angle. The angle of rotation of the torsion-head 

 was read by telescope and scale, using the same scale on 

 which the torsion deflexions and electromagnetic deflexions 

 were read. From these readings the moment actually exerted 

 by the electromagnetic efteci can be calculated and compared 

 with that demanded by theory. 



The following is a specimen set of observations : — 



Charging E.M.F E 1 = 828 volts, "effective." 



Induced E.M.F E 2 = '490 volt, 



Capacity C =9 9 x 10" 10 farad*. 



Moment of torsion of upper suspension <f> = b&2 dyne cm. 



per radian. 

 Scale distance D = 388 cm. 





A. 



B. 



A-B. 



Torsion 

 deflexions. 



6-32 

 642 



6D8 



443 

 476 



4 00 



1-94 



1-49 

 1-29 





6 30 



540 



1-14 



Electro- 



678 



5-10 



1-43 



magnetic 



6-28 



5-30 



112 



deflexions. 



6-56 



540 



113 





651 



5 40 



100 



Torsion 

 deflexions. 



629 

 R-24 

 615 



494 

 476 



4-60 



1-32 



144 

 1-44 





5-92 



4 74 



112 



Electro- 



5-80 



500 



1-00 



magnetic 



620 



512 



113 



deflexions. 



6 3i ) 



5-16 



120 





6-43 



525 



1-08 





623 



496 



1-41 



Torsion 



651 



5-67 



•92 



deflexions. 



6G7 



561 



115 ' 





686 







