Magnetic Effect of Electric Convection. 



Xft 



disk which will amount to a current equal and opposite to 

 that carried on the disk. Also, if the dram-ends are removed, 

 the full effect will not be observed unless the insulation of 

 the sectors on the mica and on the disk is good. Rowland, 

 with a similar apparatus, obtained the full effect, whilst 

 Oremieu obtained only a small part of it. This may be taken 

 to indicate that the insulation of Rowland's sectors was good 

 enough, whilst Cremieu's sectors were not well insulated. 



In fig. 2, A is the drum-end, B 1? B 2 , &c. are the sectors on 

 the mica, and C l5 C 2 , &c. the sectors on the rotating disk. 

 ;Some of the lines of force are also drawn. Consider the 



Fir-. 2. 



\ 







B £ - 



B 3 B 4 B 5 



c. 



1/ 



'III 



IIIL 



+ 



lmuiuiini/uiaiiii 



C3. »*» > C4 C 5 



sectors Ci, B 1? and B 2 . As C 1 moves along from left to 

 right, the lines of force have to get across the gap between 

 B, and B 2 . To do this, each line will curl upwards and join 

 on to the drum-end A, so that two lines will be formed, one 

 starting on Cj and ending on A, and another starting on A 

 and ending 011 B 1# The line from ( \ to A will then move 

 on and form two more, one from C x to B 2 , and another from 

 B 2 to A. The short lines from A to the sectors B 1? B 2 , &c, 

 will move along between the sectors B l5 &c. and the drum- 

 end A, and those from the one end of a sector will neutralize 

 those from the other end. 



The motion of these short lines will not produce any 

 magnetic field, because their positive and negative ends move 

 along together. Also the positive and negative ends of the 

 long lines from A and B 1? &c, to C l5 &c, move along 

 together, so that they constitute two equal and opposite 

 parallel currents, and should produce very little magnetic 

 disturbance outside the drum. Thus in this case there 

 should be no magnetic field outside the drum, which is what 

 Cremieu found to be the case. 



Fig. 3. 



^2 fc3/>T \ B 4/^N PS/^n Be 



c, 



C 2 C 3 C 4 C 5 C 6 



Now consider the case where the drum-ends are removed 

 (fig. 3). In this case the lines of force can only get across 

 the gaps by leaving short lines, e.g. from B 2 to B 1# These 

 short lines will disappear by neutralizing each other behind 



L 2 



