458 



Prof. D. E. Hughes. 



[May 27, 



ribbon of iron having the same coefficient of mutual induction as 

 copper, and it no longer behaves as a magnetic body. 



Table I shows the difference in the mutual induction of iron wires 

 and strips ; it will be seen that when the iron is in the form of a 

 wire the circular magnetism produces a marked difference between 

 iron and copper, whilst in the form of flat strips the iron resembles 

 copper (as regards mutual induction) in every respect. 



Influence of Circular Magnetism. 



I have shown in my late paper that an iron conductor composed of 

 numerous fine stranded wires (as in a rope) behaves like copper, and 

 this I regard as entirely due to the breaking up or prevention of 

 circular magnetism; I also showed a phenomenon which I could not 

 then explain, viz., that when an iron wire was heated to a yellow-red 

 heat it lost its previous high specific inductive capacity and behaved 

 like copper. The effect of heat upon magnetism is well known, con- 

 sequently we can readily explain the fall in its inductive capacity by 

 the disappearance of its circular magnetism. I found also that in 

 thin flat strips of iron there was no change whatever in its inductive 

 capacity at the yellow-red heat ; this can now be readily explained : it 

 behaved like copper when cold, and, having but little circular 

 magnetism to destroy, there was no appreciable change produced, 

 except that due to the extra resistance caused by the increased 

 temperature of the strip, and to which copper and iron are almost 

 equally sensitive, consequently we may consider iron (when in the 

 form of thin flat sheets) to behave like non-magnetic metals through- 

 out all temperatures. 



Influence of 8 elf -induction on the Resistance of the Conductor. 



A phenomenon of great importance is the effect which I have 

 observed of the resistance of a wire being greater during the rise of 

 the current, as in the variable period, than that measured or known 

 during the constant flow, as in the stable period ; by resistance I 

 mean a pure ohmic resistance, a resistance which can only be 

 measured, expressed, or balanced in ohms, and, whatever the cause, 

 the effect is one of pure ohmic resistance. 



We can imagine that at the first moment of contact there is no 

 current flowing through the wire, its resistance is then infinite, but 

 the current gradually increases in force until it arrives at its 

 maximum, as in the stable period. We have thus between the 

 moment of contact and period of steady flow a variable period 

 wherein the resistance falls in the form of a curve from infinity to its 

 well-known stable resistance ; the telephone is unable to give the 



