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On the Principles of Electro- Magnetical Machines, by Professor Jacobi, 

 of St. Petersburgh* 



" I have the honour to present to the British Association an historical 

 sketch of the laws which regulate the action of Electro-Magnetic 

 Machines, laws which will enable us to determine in a precise manner 

 the important question, of the application of this remarkable force 

 as a moving power. Since the commencement of my labours, which 

 had partly a purely practical tendency, I proposed to myself to fill 

 up as much as possible the blank which still remained in our knowledge 

 of electro-magnetism. With the assistance of M. Lenz, I prosecuted 

 the labours, which were the more arduous as they had but few 

 precedents in the direction which I considered it necessary to follow, 

 and we began to examine carefully the laws of electro-magnets. The 

 report, which contains the results of our researches, was read in June 

 1838, before the Academy of Sciences, at St. Petersburgh, I take the 

 liberty of repeating here very briefly, the contents of this first report. 

 The problem which we sought to determine may be stated as follows : 

 If a nucleus of malleable iron and a voltaic battery of a certain 

 surface is given, into what number of elements should this surface 

 be divided? what should be the thickness of the wire of the helix which 

 surrounds the nucleus? and, lastly, what number of turns should 

 this helix have, in order to produce the greatest amount of magnetism ? 

 I will not dilate here upon the manner in which we have proceeded, or 

 upon the degree of certainty which belongs to the laws established 

 according to our observations. I take the liberty of appending to this 

 statement the report in question, and will proceed to explain the parti- 

 cular laws: 1st. The amount of magnetism engendered in malleable 

 iron by galvanic currents, is in proportion to the force of those currents. 

 2ndly. The thickness of the wire twisted into a helix, and surrounding 

 a rod of iron, is absolutely of no consequence, provided that the helix 

 have the same number of turns, and the current be of the same force. 

 This law extends also to the case in which ribbons of copper are 

 employed instead of wire. Nevertheless I must notice, that in order to 

 obtain a current of equal force, it is necessary to employ a voltaic appa- 

 ratus of greater force, if small wires which offer a greater resistance are 



* These observations are referred to, by Dr. Taylor the translator of the account 

 of an electro-magnetic engine reprinted in our last number, vide, p. 119. 



