194 Sir Frederick Bramwell [April 18, 



varying amount of electrical energy for it to convert into the welding 

 current. 



I have already, when dealing with the weld itself, called your 

 attention to the important part played by the increase in electrical 

 resistance due to increase in temperature, but I now wish you to see 

 how valuable this increase of resistance is when considered in relation 

 to the question of obtaining uniformity of temperature over the whole 

 of the surfaces to be united. 



These surfaces, when first brought into contact, are rough, and 

 thus only a very small portion of them — that is to say, the extreme 

 prominences — come together, and, as a consequence, the current 

 which is i^assing is confined to these points. As these become 

 heated, however, by the passage of the current, they soften ; the 

 continued pressure applied by means of the screw flattens them, and 

 thus enlarges tlie area of contact. But, as I have shown you, the 

 hottest parts are the worst conductors, and thus the greater quantity 

 of electricity passes through the less hot parts of the enlarged area 

 of contact, raises their temperature, and the flattening of these by the 

 continued pressure causes further surfaces to come together, till all 

 are in contact ; while the current, still seeking out the coolest parts 

 as offering less resistance to its passage, raises their temperature 

 until a uniform heat and a uniform resistance are established, and 

 then this heating goes on still increasing as the current is continued, 

 in consequence of the increased electrical resistance due to the 

 increased temperature. 



I think you will agree with me that this increase of electrical 

 resistance, which follows from the rise of temperature, is most 

 valuable in enabling the necessary welding heat to be produced by 

 the passage of an electric current. 



You will observe presently, when we work the machine, that 

 the careful operator, in order to avoid burning the small surface 

 of the prominences which first come into contact, takes great care 

 to apply the current very gently. 



I will now ask to have a piece of cast steel welded to another 

 piece of cast steel. I should explain that the machine has been put 

 on to a turn-table, with the object of moving it round to face different 

 parts of the room, so as to afford a better view to the audience. I do 

 not know how many of you are aware of the difficulty of welding 

 cast steel as compared with the welding of iron. This difficulty 

 depends largely on the amount of carbon in the steel. An extremely 

 mild steel, such as is used for gun making, is easily weldable, contain- 

 ing as it does only a very low percentage of carbon ; while tool steel, 

 containing over one per cent, of carbon, presents great welding 

 difficulties. We have here two pieces of tool steel, which it would 

 be almost impossible to weld in the ordinary manner. The humming 

 noise which you hear when the current is turned on to the machine is 

 produced by our coil (our money changer) at work. 



I may tell you that I shall have to talk about a number of welds 



