DISTINCTION OF MAGNETISM AND MAGNETO-E EECTRIC INDUCTION. 191 
put over the place of the former pole, so that contrary poles were above and 
below ; on revolving the plate, the electricity was more powerful than before. 
The latter magnet was then turned end for end, so that marked poles were 
both above and below the plate, and then, upon revolving it, scarcely any elec- 
tricity was procured. By adjusting the distance of the poles so as to correspond 
with their relative force, they at last were brought so perfectly to neutralize 
each other’s inductive action upon the plate, that no electricity could be ob- 
tained with the most rapid motion. 
249. I now proceeded to compare the effect of similar and dissimilar poles 
upon iron and copper, adopting for the purpose Mr. Sturgeon’s very useful 
form of Arago’s experiment. This consists in a circular plate of metal sup- 
ported in a vertical plane by a horizontal axis, and weighted a little at one 
edge or rendered excentric so as to vibrate like a pendulum. The poles of the 
magnets are applied near the side and edges of these plates, and then the 
number of vibrations, required to reduce the vibrating arc a certain constant 
quantity, noted. In the first description of this instrument * it is said that 
opposite poles produced the greatest retarding effect, and similar poles none ; 
and yet within a page of the place the effect is considered as of the same kind 
with that produced in iron. 
250. I had two such plates mounted, one of copper, one of iron. The copper 
plate alone gave sixty vibrations, in the average of several experiments, before 
the arc of vibration was reduced from one constant mark to another. On putting 
opposite magnetic poles near to, and on each side of, the same place, the vibra- 
tions were reduced to fifteen. On putting similar poles on each side of it, they 
rose to fifty; and on putting two pieces of wood of equal size with the poles 
equally near, they became fifty-two. So that, when similar poles were used, the 
magnetic effect was little or none, (the obstruction being due to the confine- 
ment of the air, rather,) whilst with opposite poles it was the greatest possible. 
When a pole was presented to the edge of the plate, no retardation occurred. 
251. The iron plate alone made thirty-two vibrations, whilst the arc of vibra- 
tion diminished a certain quantity. On presenting a magnetic pole to the edge 
of the plate (247-), the vibrations were diminished to eleven ; and when the 
pole was about half an inch from the edge, to five. 
* Edin. Phil. Journal, 1825. p. 124. 
