M. A. Berlin on Circular Magnetic Polarization. 483 



The same condition is still better satisfietTin Rwhmkorff's* 

 apparatus, where the helices themselves are pierced in the direc- 

 tion of their axes. The Ecole Normale has such an apparatus, 

 54; kilogrammes in weight, which is as powerful as that be- 

 longing to M. Becquerel, although it is not more than one- 

 third its weight. 



The effect produced by an electro- magnet of the same shape 

 depends on the mass. Thus, an apparatus one-fifth the size 

 of the preceding, produced, under the same circumstances, 

 rotations twice as feeble. 



The dimensions of the wire have also a certain influence. In 

 general it should be thick. That used in the Ruhmkorfi^ ap- 

 paratus is 2*50 millims. in diameter ; and in M. Becquerel's 

 electro-magnet the maximum effect is obtained by doubling 

 the section of the conducting wire. With respect to the mass 

 employed, the instrument-makers at present are in the habit 

 of rolling around the iron nucleus a thickness of wire equal 

 to its radius, so that the external diameter of the reel is twice 

 that of the interior cylinder. 



Lastly, the intensity of the phaenomenon likewise depends 

 on the current, or rather on the relation existing between the 

 dimensions of the electro-magnet, and the force of the battery 

 exciting it; so that a very powerful apparatus may appear 

 very feeble when it is not set in action by a battery of suffi- 

 cient strength. With the same apparatus the intensity of the 

 effects produced increases with the intensity of the current; 

 as to this, it increases with the number of the elements of the 

 battery, but is far from being proportional to it. Again, when 

 the current has a certain force, it is more advantageous to 

 increase the quantity of electricity than the tension ; that is to 

 say, it is better to enlarge the surface than the number of the 

 elements of the battery. It is therefore desirable to ascertain 

 what is the arrangement for a given battery which will pro- 

 duce the maximum effect. Thus, having at my disposal 80 

 Bunsen's elements, I have found that the best arrangement 

 to be given to them in order to excite the great Ruhmkorff 

 apparatus, was to join, by poles of the same name, four bat- 

 teries of 20 elements. The results just stated follow from the 

 subjoined table, which contains the rotations observed in one 

 of Faraday's flint-glasses of 39 millimetres in length placed 

 between the two poles of the Ruhmkorff apparatus. 



* Ann. de Chim. et de Pht/s., third series, vol. xviii. p. 318. 

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