16 Popular summary of Experiments on tlie 



cited or disturbed by rapid rotation, and to pursue this 

 inquiry, the following experiments were undertaken. 



They were begun in December 1824, and completed last 

 January, but the publication of them was delayed till June, 

 in order that Mr Christie's paper might appear at the same 

 time, the similarity of the inquiries rendering this measure 

 desirable. During this interval, M. Arago discovered the mag- 

 netic effect of copper and other metals while in rotation ; a 

 subject which we shall have occasion to resume in our abstract 

 of Messrs Babbage and Herschefs experiments. 



Mr Barlow's first experiments were made on a thirteen inch 

 shell, attached to one of the lathes in the Royal Arsenal, 

 turned by the steam-engine, the mean speed of which was 

 about 640 revolutions per minute. With this the effect of 

 velocity was at once rendered obvious, — the deviation of the 

 needle increasing with the speed, — and it remained constant 

 in all cases when the velocity was constant ; but the needle 

 always returned correctly to its proper or original direction, 

 the moment the motion of the ball ceased. This, therefore, 

 is a phenomenon different from that observed by Mr Christie. 

 In the latter, the effect is permanent, and independent of 

 the number of revolutions ; while, in the former, it is tem- 

 porary, and is altogether dependent on the rapidity of rota- 

 tion. 



Mr Barlow afterwards suspended an eight inch shell after 

 the manner of the cylinder of an electrical machine, and 

 having the means of placing the axis of rotation in different 

 azimuths, he examined, by neutralizing the needle, the direc- 

 tion of the new force thus impressed upon the shell, which 

 he found in all cases equivalent to a polarization at right 

 angles to the axis of rotation ; and the explanation he has 

 given is nearly the same as we have ventured upon in the 

 preceding experiments, viz. that, if the iron were perfectly 

 soft, the magnetic fluid would maintain its natural polarized 

 position; but this not being the case, it is carried forward 

 by the rapid rotation of the ball, thereby giving a certain de- 

 gree of obliquity to the general axis of polarization, which 

 oblique force, being resolved into two forces, one of them in 

 the natural direction of the needle, the other perpendicular 



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