100 On the Magnetism devoloped by Rotation. 



plate ; on causing the spindle and plate to revolve, the needle 

 was considerably deflected, and more and more as the velocity 

 was increased ; so that, when the plate was put into rapid ro- 

 tation, the needle also began, after a few vibrations, to re- 

 volve, and at length with considerable velocity. 



" 1. In order to repeat this experiment, I connected the 

 wheel of my turning lathe with a vertical spindle, which I could 

 make revolve forty-five times per second ; and on this I placed 

 a thin copper plate, about six inches in diameter, and over 

 this a needle about five inches long, shut up in a close box, 

 about one inch, or rather less, above the plate. When putting 

 the lathe in motion, I found it to deflect the needle about five 

 points, the deflection being always in the same direction as 

 the motion of the plate, but we could not cause it to revolve. 

 The needle was, therefore, partly neutralized by a bar mag- 

 net, and the experiment repeated. We then very soon ob- 

 tained a considerable I'otatory motion in the needle ; and, by 

 usin^ a larger and heavier plate, the same was producetl after- 

 wards 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 ori- 

 ginating with M. Arago, at least that I repeated, was the 

 rotation of a plate cut into radii like a star, which was said, as 

 I understood, 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 quantity of copper thus taken away. 



" i. I now tried a zhic 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 

 considerably greater than with the copper plate. 



*' 6. The copper plate was again replaced, and a brass needle 

 placed in tlie box. Some motion was obtained, but it was 

 very equivocal, so that I cannot venture to say that it was cer- 

 tainly due to the rotation. 



" 7. A heavy horse-shoe mag-net was now suspended by a 

 line from the ceiling; and it was put in rotation by the revo- 

 lution of the copper plate, a paper screen having been first 

 interposed between them. 



" 8. One copper plate was suspended over another, but no 

 motion was obtained ; and the same took place when the cop- 

 per plate was suspended over an iron one. 



« 9. A bar 



