818 The Outline of Science 



now smelt iron and convert it into high-quality steels by the hun- 

 dred thousand tons. The furnaces are in principle enlarged edi- 

 tions of the arc lamp and domestic "resistance" heater; and all 

 taken together they consume millions of electrical horse-power 

 continuously. 



A very important branch of electric heating is that concerned 

 with welding. The electric current has given the engineer an 

 invaluable ally when dealing with repairs to heavy machinery. 

 A typical instance of its helpfulness is supplied by what hap- 

 pened during the war when enemy vessels, interned in American 

 ports, had their boilers and engines rendered useless before aban- 

 donment. Under old conditions the only remedy would have 

 been to replace the injured parts by new ones; but with the aid 

 of the electric arc it was possible to unite the edges of broken 

 pieces, and affix patches so securely as to render broken ma- 

 chinery as good as new in effectiveness, if not in appearance. 

 The result was in a very short time many large ships that the 

 enemy regarded as definitely hors de combat were ploughing the 

 seas again and doing useful service. 



The electric weld is replacing the rivet in many manufactur- 

 ing processes. The steel plates of a ship's skin may now be 

 joined without the use of a single rivet; and "welded" vessels 

 are actually in commission. Many articles of everyday use are 

 electrically welded. To make a "spot" or local weld, the parts 

 to be joined are pressed together between the points of bars 

 connected with an electric circuit, and the heat set up by the 

 current causes the parts to fuse and amalgamate over an area 

 between the bars. Again, suppose a short bar has to be affixed 

 to a thick plate, to project from it. Instead of drilling a hole 

 in the plate, tapping a thread in it, and cutting a thread on the 

 bar, the last is merely pressed against the plate, and a powerful 

 current passed through both parts; and in a few minutes the 

 two are practically one. 



