148 Br. H. A. WHsod on the 



the sectors B l5 &c, and will therefore produce a current equal 

 and opposite to that produced by the negative ends of the 

 long lines between the two sets of sectors. As the sectors 

 Cj, &c. move from left to right, the lines of force w r ill tend 

 to get sheared over towards the right, so that a line going for 

 example from Ci to B : will ultimately strike B 2 and form two 

 lines, one from B 2 to B 1? and one from C x to B 2 . Thus, at 

 every gap between the sectors there will be a continuous 

 production of lines going from one sector to the next across 

 the gap. One end of each sector will therefore continually 

 tend to get positively charged, and the other end negatively 

 charged. These opposite charges will of course continually 

 neutralize one another, which will constitute a current equal 

 and opposite to that produced by the motion of the negative 

 ends of the lines of force along the sectors B 1? &c. This 

 process of neutralization is shown more in detail in fig. 4, where 

 only these " short " lines and their charges are represented^ 



Fig. 4. 



^ fexX/^\^/^^ \ 



£** 



It is evident that what really happens is that the moving 

 sectors carry round their charges while a circulation of elec- 

 tricity is produced in the other sectors. Such a circulation 

 of course cannot produce a magnetic field, because as much 

 electricity must pass across each sector in one direction as in 

 the opposite direction. Thus, in this case, the motion of the 

 disk should produce a magnetic field. Cremieu, however, 

 only obtained a small field, which he supposed was a spurious 

 effect. 



It is assumed in the above that the insulation between the 

 sectors is perfect. If this is not the case, then the lines of 

 force between the two sets of sectors will slip across the 

 gaps instead of forming the short lines which produce the 

 compensating currents. Since the actual currents in these 

 experiments are in any case very small, whereas the electric 

 intensity between the two sets of sectors is very large, it is 

 easy to see that .unless the insulation of the sectors is very 

 good, the lines of force will nearly all slip across the gaps 

 where the insulation is bad, so that there will merely be two 

 nearly equal and opposite parallel currents, and consequently 

 very little magnetic field. 



It seems therefore probable that in Cremieu's experiments 

 without the drum-ends, the insulation of the sectors was not 



