Messrs. Barlow's and Marsh's Experiments. 101 



" 9. A bar magnet, rather shorter than the diameter of the 

 copper plate, was fixed horizontally to the upright spindle ; 

 and being made to revolve, the plate very soon acquired ro- 

 tation. A paper screen was, in this, as in the preceding ex- 

 periments, interposed 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 

 neutralizing the needle, and bringing either of its poles directly 

 to the plate, it then always deviated in the direction of the 

 motion of the plate; whichever pole of the needle was di- 

 rected to the former. The needle, of course, therefore, de- 

 viated different ways (all other things being the same), when it 

 was above or below the axis ; but in the direct horizontal line 

 of the axis no motion in the needle took place. 



"11. The above are the principal experiments that I assisted 

 in making by revolving the plate ; but these having suggested 

 to Mr. Barlow that all the results obtained might be explained, 

 by supposing that there existed a slight magnetic power in 

 copper, and in the various metals which had a tendency to 

 draw the needle after the plate, or the latter after the former, 

 he endeavoured to exhibit this by direct experiment, indepen- 

 dent of revolution. With this view, he neutralized a needle 

 very accurately; and then applying very near to its poles the 

 end of a round bi-ass ruler, the attiaction of the latter was ob- 

 vious, — it drew the needle several degrees, — then, withdrawing 

 it, and catching the needle again in its returning vibration, it 

 was drawn out some further degrees ; and, in a very short time, 

 the deflection was converted into a revolution, which, by al- 

 ternately presenting and withdrawing the needle, was at length 

 rendered very rapid. 



" 12. The same result was obtained by two or three different 

 pieces of brass ; but there were other pieces, although of the 

 same size and form, which had little or no effect. 



" The following experiment is due to Mr. Sturgeon, of 

 Woolwich. 



"13. A thin copper plate or wheel, about five or six inches 

 in diameter, was suspended very delicately on an axis, and 

 then one side a little weighted, in order to give it a tendency 

 to oscillate. The heavy point was now raised level with the 

 axis, and the number of vibrations the plate made before 

 it came to rest were counted. The same was again done, with 

 this difference only, that the vibrations now took place be- 

 tween the poles of a horse-shoe magnet; and the number of 

 them before the plate came to rest, was very little more than 

 one half of wluit they were in the former instance. 



"This 



