Magneto-electric Phenomenon, 



223 



strength determined by some recognized method. I therefore 

 made a small coil of twelve turns of copper wire of 1*45 

 centim. radius and with a resistance of 0*085 ohm. The 

 area of the coil was 79*4 square centim. The moment of 

 inertia of coil and index was found by comparison with a 

 cylinder to be 58'1 units, and the torsional value of the sup- 

 porting wire to be 17,000 units. The plane of the coil was 

 adjusted by the usual method to zero, and then set to 45°. 

 The currents from 10, 9, 8, &c. to 1 Grove cells were sent in 

 succession through the coils of the electromagnet and through 

 a Siemens electrodynamometer. The following Table shows 

 the amounts of the positive and negative impulses, the strengths 

 of the magnetizing currents in amperes, and the field-inten- 

 sities calculated from the positive throws by the formula 



H 2 = 931 1000 x Throw measured in degrees. 

 Table III. 



Number of 

 cells. 



Magnetizing 

 current, in 

 amperes. 



Negative 

 throw. 



Positive 

 throw. 



Calculated 

 field, in ab- 

 solute units. 



10 



11-85 



10-5 



27-5 



16,000 



9 



11 2 



9 



26 



15,600 



8 



10-44 



8-2 



24-5 



15,100 



7 



9-47 



7 



22-5 



14,500 



6 



8-63 



6 



21-3 



14,100 



5 



7-60 



4-8 



19-2 



13,400 



4 



6-53 



3-3 



16-5 



12,400 



3 



5-18 



2-2 



13 



11,000 



2 



3-67 



1 



8 



8,630 



1 



1-83 



02 



2-8 



5,070 



The ends of the coil which were soldered together were then 

 unsoldered without disturbing the soldered connexion between 

 the torsion-wire and one end. The other end was bent so as 

 to dip into a small mercury-cup in the axial line, so that a 

 known current, measured by a second electrodynamometer, 

 could be sent through the coil. During this change no part 

 of the apparatus was moved at all ; it was, however, necessarv 

 to redetermine the zero position, which was now rather more 

 difficult, for the resistance of the torsion-wire was so great in 

 comparison with that of the coil alone, that the throw at any 

 angle was only about one tenth of what it was before. As 

 before, the current from 10 to 1 cells was sent successively 

 through the coils of the electromagnet and an electrodynamo- 



