140 K H. Hall — Effect of Magnetic Force on the 



about 8 mm wide and 12 mm long. In each case the shortest 

 diameter reached nearly from arm to arm. Care was taken to 

 prevent either loop from overhanging the edge of the cross at 

 any point. The loops were made as equal circles by winding 

 them upon a cylinder about l cm in diameter. They must have 

 had nearly equal areas in their final forms. After somewhat 

 careful measurements the final area of each was estimated at 

 0*78 sq. cm. This estimate may be wrong by several per cent, 

 for either or both loops. 



The plan was to put in turn Nos. 5 and 7 between the poles 

 of the electromagnet and compare, by means of a ballistic 

 galvanometer, the induction currents produced in the loops by 

 suddenly reversing the direction of the magnetic field. It is 

 believed that the ratio of these two induction currents does not 

 differ many per cent from the ratio which the intensity of 

 magnetic induction through the effective portion of one cross 

 bears to that through the effective portion of the other cross. 



Comparisons were also made between the induction cur- 

 rents obtained with each loop and those obtained with a coil of 

 four circular turns having a total area about 5 # 24 sq. cm., this 

 Jatter coil being the one used to determine the strength of the 

 field when no magnetic metal was between the poles. This 

 coil will be called the test-coil. Its ordinary use was not to 

 remain between the poles and give an induction-current upon 

 reversal of the magnet-current, but to be withdrawn suddenly 

 from the field while the magnet remained in full force. This 

 latter use is of course the better one when it can be employed, 

 for it cannot always be assumed that the visible induction 

 effect obtained by reversal will indicate the true change of 

 strength of the field. As this particular matter had to be 

 investigated with some care, I will give the results, although 

 the following table does not attempt great accuracy. 



M= intensity of field in c. g. s. units. 

 d =swing of needle on pulling test-eoil out of field. 

 ])= swing of needle on reversing magnet current, test-coil 

 remaining between poles. 



M 



d 



2xd 



D 



2300 



C '644 



l°-29 



1°-16 



4800 



1-35 



2-70 



2-43 



8900 



2-52 



5-04 



4-87 



8100 



2-32 



4-64 • 



4-38 



8500 



2-43 



4-86 



4-80 



8700 



2*51 



5*02 



4-83 



9200 



2-61 



5-22 



5-13 



9200 



2-04 



5-28 



5-20 



9250 



2-62 



5-24 



5-14 



9700 



2-82 



5-64 



5-54 



