PROFESSOR THOMSON ON THE EEECTRO-DVNAMIC QUAIHTIES OF METALS. 749 
square to which the copper slip is attached. A copper wire, to serve as fixed galva- 
nometer electrode, was soldered to the lead band, at a point in the middle of its 
breadth close to its edge of attachment to the iron. A copper cradle was put between 
the flat poles of the electro-magnet, as before (see above, § 161), and covered with 
a piece of paper. The iron square was supported upon it in a position with the 
line joining the poles perpendicular to the line of the current through it. Then, the 
current being kept steadily flowing through the iron and lead band, a zero-point 
was found on the testing conductor, and a multiplying branch (one yard of No. 18 
copper) was soldered with its ends ^ of an inch on each side of this point, in the 
usual way. The zero-point on this multiplying branch was almost immediately 
found, and continued on the whole very steady from the first. The galvanometer 
circuit being broken, a magnetizing current from six small iron cells was sent through 
the coils of the electro-magnet, and the needle of the galvanometer was let settle (as 
it could be in a few seconds by the aid of a little magnet held in the hand) into its 
position of equilibrium as affected by the direct force of the magnet. On completing 
the galvanometer circuit again, with its moveable electrode on the same point of the 
multiplying branch as before, a current was made sensible by an excessively slight 
deflection. The galvanometer circuit being broken, and the electro-magnet reversed, 
a similar deflection was found in the galvanometer on again completing its circuit. 
It ceased, as nearly as could be discovered, when the electro-magnet was unmade, 
and was uniformly observed when the magnet was made again either way, in a great 
many repetitions. The current indicated by the galvanometer when the magnet was 
made was always such as to be corrected by carrying the moveable electrode from 
its previous zero-point, along the multiplying branch, towards the part of the testing 
conductor terminating at the iron square, and therefore indicated an increase of con- 
ductivity in the iron. The effect was so very slight, that I could scarcely determine 
how much the moveable conductor had to be shifted to correct it. I intend to repeat 
the experiment with similar arrangements, but with two or three times as powerful a 
current through the electro-magnet, which ought to give about four or nine times 
the amount of effect. In the mean time, however, I am quite convinced that I have 
observed the true result, and I conclude that the electric conductivity of iron is 
increased hy magnetic force across the lines of current. 
176. Exp. 8. To show the variation of a line of electric equUihrium in a circular disc 
of iron conducting electricity between two opposite points of its circumference, when sub- 
jected to magnetic force in a direction at an angle of 4ib° to the line joining these points. 
— A circle 2’3 inches diameter was cut from a piece of sheet iron, and ground down 
to a thickness which must have been about g-gth of an inch, as the prepared disc was 
found to weigh 114 grains. Two stout copper electrodes were soldered to its circum- 
ference at opposite points. A point at 90° on the circumference from one of these 
was taken, and at about :^oth of an inch on each side of it were soldered the ends of 
a piece of No. 18 copper wire two yards long, to serve as a multiplying branch. The 
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