induced or exhibited in Iron , fyc. by Rotation . 123 
by using a larger and heavier plate, the same was produced af- 
terwards without neutralizing the needle. 
2. Another experiment, which was mentioned as one of M. 
Arago’s, and which I repeated, was, by interposing a plate of 
iron between the copper plate and the needle. In this case, no 
effect could be produced on the needle by the rotation of the 
copper plate, the iron clearly intercepting the action. 
3. The only other experiment that I am aware of, as origi- 
nating with M. Ampere, at least that I repeated, was the rota- 
tion of a plate cut into radii like a star, which was said, as I un- 
derstood, to produce no effect : this, however, was not the case 
in my experiments, — it certainly produced a less effect, but, I 
think, not less than might have been anticipated, from the quan- 
tity of copper thus taken away. 
4. I now tried a zinc plate instead of a copper plate, and the 
effect was nearly the same as before, but a little less. 
5. An iron-plate was now substituted, and the effect was con- 
siderably greater than with the copper plate. 
6. The copper-plate was again replaced, and a brass needle 
placed in the box. Some motion was obtained, but it was very 
equivocal, so that I cannot venture to say that it was certainly 
due to the rotation. 
7. A heavy horse-shoe magnet was now suspended by a line 
from the ceiling ; and it was put in rotation by the revolution of 
the copper plate,- — a paper screen having been first interposed 
between them. 
8. One copper plate was suspended over another, but no mo- 
tion was obtained ; and the same took place when the copper 
plate was suspended over an iron one. 
9. A bar-magnet, rather shorter than the diameter of the cop- 
per plate, w r as fixed horizontally to the upright spindle ; and be- 
ing made to revolve, the plate very soon acquired rotation. A 
paper-screen was, in this, as in the preceding experiments, inter- 
posed between the plate and magnet. 
10. The plate was now applied immediately to the axis of the 
lathe, so as to cause it to revolve vertically, and the needle 
placed near to it ; but no motion took place, till, by nearly neu- 
tralizing the needle, and bringing either of its poles directly to 
the plate, it then always deviated in the direction of the motion 
