292 The Eev. T. R. Robinson's Experimental Researches on the 



action, and the capability of being moved to any locality where a couple of 

 wires can be led from its battery, deserve special consideration. Such views 

 several years ago induced my friend, Mr. T. F. Bergin, to experiment on the 

 construction of a machine suitable to the workshop of the amateur, or the labo- 

 ratory of the philosopher ; and I hope he will at no distant period lay his in- 

 vention before the Academy. In its progress he occasionally consulted me as 

 to the form and mass of the magnets to be employed ; the distribution and kind 

 of wire in their helices ; and the intensity of the currents transmitted through 

 them which might be expected to give the highest dynamic effect from a given 

 consumption of materials. On all these points I was surprised to find that 

 there was httle or no exact information extant ; I therefore determined to look 

 for it myself; and since the beginning of 1848 have given to this object such 

 attention as was permitted by my other avocations. In carrying it out I have 

 derived much valuable aid from Mr. Bergin, not merely in the contriving and 

 constructing the necessary apparatus, but also in making many experiments 

 which I had not the means of performing. During this period several German 

 physicists have been engaged in similar investigations ;* but if I do not deceive 

 myself, neither their results, nor those of Mr. JouLE,f go so far as to make the 

 present communication unnecessary ; and I trust it will be found not merely 

 useful to the practical magnetician, but also valuable, as affording data which 

 have been carefully determined, to those, who like Dr. William Thomson, are 

 investigating the theory of magnetic induction. 



Before describing my methods of experimenting, a brief account of what 

 occurs in the action of the electro-magnet may make then- object more intel- 

 ligible. If we conceive the cylindric core divided into thin sections perpendicu- 

 lar to its axis, and confine ourselves to the uppermost of them ; on passing a 

 current through the helix, its two sui-faces will possess opposite polarities, de- 

 rived mainly from the inducing power of those spires which are in its plane, but 

 also in a decreasing amount from those which are below it. The intensity of 

 these polarities depends on that of the inducing forces and of those which op- 

 pose them ; the former is known to be proportional to the intensity of the 



* Translated by Dr. Tvndall in the PhUosopliical Magazine, March, 1851. 

 t Philosophical Magazine, October, 1851. 



