302 



Miscellaneous. 



collecting the same number of coils around the extremities. The dif- 

 ferences between the observations and the simple laws are, as will 

 be judged, quite inconsiderable for practical purposes, and will, in time, 

 I hope, entirely disappear by a complete integration embracing the 

 whole length of the rods, and founded upon the effect of an elementary 

 part of the current. I will now hasten on to the immediate object 

 of my present address. In March 1839, M. Lenz and I presented to the 

 Academy of Sciences at St, Petersburgh, a report, which I shall present 

 to the Association. It contains the result of the experiments by which 

 we have been enabled to establish the remarkable law, that the attraction 

 of the electro-magnets is proportional to the square of the force of the galvanic 

 current, to the influence of which the rods of iron are submitted. This law is 

 of the highest practical importance, as it serves for the basis of the 

 whole theory of electro-magnetic machines. 



" Before proceeding, I may be permitted to make some remarks con- 

 cerning an instrument which I laid before the Academy of Sciences, in 

 the commencement of this year. It is destined to regulate the galvanic 

 current, and is of value in many investigations of this kind. During my 

 sojourn in London, Prof. Wheatstone has shown me an instrument, 

 founded on exactly the same principles as mine, and with very incon- 

 siderable modifications and differences. Now, it is quite impossible that 

 he should have had the least notice of my instrument ; but as it is pro- 

 bable that its use may be greatly extended, I must add, that while I 

 have only used this instrument for regulating the force of the currents, 

 he has founded upon it a new method of measuring these currents, and 

 of determining the different elements or constants, which enter into the 

 analytical expressions, and on which depends the action of any galvanic 

 combination. It is principally to the measure of the electro-motive force, 

 by those means, that Mr. Wheatstone has directed his attention ; and 

 he has shown me, in his unpublished papers, very valuable results which 

 he has obtained by this method. 



" While these purely theoretical researches were in progress, I did not 

 fail myself to enter directly upon the question of the practical application 

 of electro-magnetism. Unfortunately, I cannot here give the details either 

 of the experiments which I have made upon a very large scale, or of 

 the machines and apparatus of various kinds which I have constructed. 

 The necessity of multiplying the facts or tangible results — a necessity 

 the more urgent, because the practical applications of this force in- 

 creased so very rapidly — this necessity, I say, has not allowed me time 

 or leisure to digest and arrange them. I can only here express my 

 readiness to afford any explanation of the details which may be desir- 



