Miscellaneous. 



299 



employed. 3rdly. If the current remain the same, the influence which 

 the diameter of the helix exercises may be neglected in the majority 

 of practical cases. 4thly. The total action of the electro-magnetic helix 

 upon the rod of iron, is equal to the sum of the effects produced by 

 each coil separately. Adopting these laws, and submitting them to 

 calculation according to the formula of M. Ohm, the importance of 

 which formula was but lately begun to be appreciated by some British 

 philosophers, we have established the formula which contains all the 

 particular conditions required to obtain the maximum amount of mag- 

 netism, which may be expressed in the following extremely simple man- 

 ner ; viz. the maximum of magnetism is always obtained when the total resist- 

 ance of the conducting wire, which forms the helix, is equal to the total resist- 

 ance of the pile. On referring to the remarkable law of the definite 

 action of the galvanic current, established by Mr. Faraday, it is found 

 that the magnetism of malleable iron divided by the consumption 

 of zinc, — a quantity which we have called economic effect, is with 

 reference to the maximum of this magnetism, a constant, or an ex- 

 pression into which neither the thickness of the wire nor the number of 

 the elements into which the total given surface of the battery is divided 

 enters, but only the total thickness of the envelope. 



" Having finished these first researches, and having obtained these re- 

 sults, which were highly satisfactory, not only for their simplicity, but 

 also for their practical value, we set about extending our inquiries to 

 iron rods of different dimensions. Is there, it may be asked, any speci- 

 fic effect produced by the length or thickness of the nucleus ? or does the 

 degree of magnetism solely depend upon the construction of the helix, 

 and the force of the current? The solution of this new problem presents 

 a greater difficulty than the problem which we had succeeded in com- 

 pletely solving. Now, we are obliged to take iron rods of different 

 dimensions, and, consequently, in all probability of different qualities. 

 Similar conditions with reference to the action of the electro-magnetic 

 helices are likewise difficult to obtain ; and we soon perceived that these 

 circumstances rendered it impossible to attain so close an accordance, 

 as that which we had obtained in our former observations. Although 

 these experiments were made two years ago, the results have not yet 

 been published, because, being occupied with other labours, we have not 

 been able to find the necessary time for their reduction and arrangement, 

 and for the requisite calculations. Nevertheless I take the liberty of 

 presenting to the Section some results, which are not devoid of interest, 

 and which are intimately connected with the question of electro-magne- 

 tic machines. We submitted nine cylinders of malleable iron, each eight 



