188 Sir Frederick Bramwell [Aj^ril 18, 



common term, its electromotive force. It also depends upon the 

 electrical resistance of the material through which the current is 

 passing. This resistance is very different in various substances, and 

 varies even in the same substance under varying conditions, as I shall 

 show you hereafter. 



You have all seen over and over again the experiment of heating 

 a wire which seemed, before heating, to be the same from end to end, 

 that is to say, all of one diameter and of one appearance ; but on 

 passing an electric current through it you found that it was made up 

 of two different materials ; for while it was white-hot in alternate 

 sections, it was dull in the intermediate portions. Those parts which 

 remained dull did so because they were made of a metal (probably 

 of Silver) which allowed the electric current to pass without much 

 opposition, as compared with the parts which glowed, these being 

 probably of Platinum, this metal offering a greater resistance to the 

 current, and thus generating greater heat. I have placed in the 

 "jaws" or " holders" of the electric welding machine before you, a 

 compound wire made of a length of copper, a length of iron, and a 

 length of German silver, or, as a matter of fact, the German silver is 

 between the other two, all being of equal diameter. On passing the 

 electric current through, I trust you will find that the German silver 

 becomes hotter than the iron, because the resistance of an equal sec- 

 tional area of it is in round numbers double that of iron, and that the 

 copper does not apparently become hot at all, because its resistance 

 is only one-sixth that of the iron, or one-twelfth that of German 

 silver ; in stating these proportions I am referring to ordinary atmo- 

 spheric temperature, for when metals are heated an entirely new 

 set of resistances come into play, these varying considerably with 

 variations in their temperature. That upper arch is the German 

 silver. It is, as you see, very hot ; the left-hand portion is less hot, 

 and the right-hand part is apparently unheated. Those metals are, 

 as I have said, iron, German silver, copper. 



I told you just now that the electrical resistance of metals and of 

 other bodies alters with their temperature. This alteration is different 

 with different metals ; but in the case of the metal with which I am 

 concerned to-night — iron — the variation is very considerable. Many 

 persons have studied this question of the changes of resistance due 

 to the increase of temperature, and among them Dr. Hopkinson. He 

 has kindly furnished me with the results of his experiments, and I 

 exhibit them to you on this diagram in the form of a curve. 



From this we find that if the resistance of iron at 32^ Fahr. be 

 taken as unity, at 1832^ Fahr. the resistance has gone up to over 

 eleven times. You see the way in which it rises, and the peculiar 

 kink there is in the curve at about 1400^ or 1500^, when the resist- 

 ance is about ten times what it is at 32^. This fact of the lars^e 

 increase of electrical resistance with the increase of temperature is a 

 matter of the utmost importance in welding by electricity, as I hope 

 to show you later on. 



