463 On the Self-induction of an Electric Current. [May 27, 



a solid bar, whilst the resistance in the variable period fell from 275 

 to but 69 per cent. 



It is well known that a bundle of fine wires magnetises quicker than 

 a solid bar, and this may have had its effect, although not more 

 important than the suppression of the eddy currents, as the latter may 

 be the cause of the former. The table shows that where we have 

 suppressed these currents as far as possible by introducing a core of 

 insulated wires containing less iron than in the previous experiment 

 but fully insured against eddy currents, the induction was the highest 

 and the resistance the lowest yet found for a helix containing an iron 

 core ; the experiment proves to my mind that the extra resistance 

 found in the three last experiments is due both to the electromagnetic 

 inertia of the eddy currents, and the inertia of the magnetic molecules 

 of iron. 



The experiments related in this paper have been most carefully 

 made and verified, and from the ease and certain action as well as the 

 invariable results obtained by the method which I have described, 

 they should be easy to repeat by others. 



If we regard the whole of these researches we cannot fail to notice 

 certain important laws which act in the greater portion of them. I 

 have shown : 1st. That the contiguous portions of the same current 

 react upon each other in the interior of its own portion of the con- 

 ductor similar to their known exterior reactions on separate portions 

 of the same conductor. 2nd. That the coefficient of mutual induction 

 is less in iron than copper wires, but that their coefficient is the same 

 when the conductor is in the form of a ribbon. 3rd. That the induc- 

 tive capacity of different metals depends on their specific resistance, 

 on their electromagnetic capacity for circular magnetism, and on the 

 geometrical form of their conductors. 4th. That the inductive 

 capacity of a conductor of magnetic metal is dependent upon the for- 

 mation of circular magnetism and not upon its internal magnetic 

 permeability. 5th. That a magnetic metal can be rendered equally 

 free from circular magnetism as the non-magnetic metals. 6th. That 

 we have experimental evidence of electromagnetic inertia and the 

 deleterious effects of eddy currents in the cores of electromagnets. 



In addition to the above effects, we have the discovery of a large 

 increase in the ohmic resistance during the variable period, allowing 

 us to demonstrate and measure the gradual rise of an electric current 

 in its conductor. 



In conclusion, I desire to express my warmest thanks to Lord 

 Rayleigh, Mr. F. L. Pope, Professor Forbes, Dr. Hopkinson, Mr. W. H. 

 Preece, Dr. Fleming, Mr. Fitzgerald, Professor Silvanus Thompson, 

 and Prof essor Ayrton, for the important theoretical contributions they 

 gave on the discussion of my first paper. Mr. W. H. Preece, Electri- 

 cian of the Post Office, not only gave information of great practical 



