1882.] Mr. Spottiswoode on Matter and Magneto-Electric Action. 75 



WEEKLY EVENING MEETING, 



Friday, March 31, 1882. 



George Busk, Esq. F.E.S. Treasurer and Vice-President, in the Chair. 



W. Spottiswoode, Esq. LL.D. Pres. R.S. M.B.I. 



Matter and Magneto -Electric Action. 



The late Professor Clerk Maxwell, in his work on 'Electricity 

 and Magnetism ' (vol. ii. p. 146), lays down as a principle that " the 

 mechanical force which urges a conductor carrying a current across 

 the lines of magnetic force, acts, not on the electric current, but on 

 the conductor which carries it. If the conductor be a rotating disk or 

 a fluid it will move in obedience to this force, and this motion may 

 or may not be accompanied with a change of position of the electric 

 current which it carries. But if the current itself be free to choose 

 any path through a fixed solid conductor or a network of wires, then, 

 when a constant magnetic force is made to act on the system, the path 

 of the current through the conductors is not permanently altered, but 

 after certain transient phenomena, called induction currents, have 

 subsided, the distribution of the current will be found to be the same 

 as if no magnetic force were in action. The only force which acts 

 on electric currents is electromotive force, which must be distinguished 

 from the mechanical force which is the subject of this chapter." 



In the investigation on electric discharges, on which Mr. Moulton 

 and myself have been long engaged, we have met with some phenomena 

 of which the principle above enunciated afiords the best, if not the 

 only, explanation. But whether they be regarded as facts arising out 

 of that investigation, or as experimental illustrations of a principle 

 laid down by so great a master of the subject as Professor Clerk 

 Maxwell, I have ventured to hope that they may possess sufficient 

 interest to form the subject of my present discourse. 



The experiments to which I refer, and of which I now propose to 

 offer a summary, depend largely upon a special method of exciting an 

 induction coil. This method was described in two papers, published 

 in the ' Philosophical Magazine ' (November 1879) and in the ' Pro- 

 ceedings of the Eoyal Society ' (vol. xxx. p. 173), respectively ; but as 

 its use appears to be still mainly confined to my own laboratory, and 

 to that of the Royal Institution, I will, with your permission, devote 

 a short time to a description of it, and to an exhibition of its general 

 effects. 



The method consists in connecting the primary circuit directly 

 with a dynamo- or magneto-machine giving alternate currents. In 

 the present case, I use one of M. de Meritens' excellent machines 



