TO SHIP CONSTRUCTION. 203 



reached the stage of development which might warrant its practical application to 

 ship construction, it offers every prospect of doing so within the very near future. 



Obvious developments with this system are multiple spot and continuous or 

 seam welding, both of which will tend towards great saving in time. 



In general spot welding is much more rapid than arc welding and requires less 

 labor, but more power and a much heavier and more expensive machine. 



(b) Butt Welding. — This system of resistance welding is especially applicable 

 to bars of uniform section and will probably be used extensively in jointing rein- 

 forcements in ferro-concrete construction. 



The bars to be welded are brought together, end on, clamped and a low-pressure 

 current passed through until a welding heat is obtained. Then end pressure is ap- 

 plied until the metal at the joint shows signs of squeezing out. A machine for this 

 type of welding has been ordered by the Emergency Fleet Corporation and will be 

 tried out during the construction of a 3,500 D. W. T. reinforced concrete steamer 

 building by the Fougner Concrete Shipbuilding Company. 



Electric Arc Welding. 



In this method of welding the electrode and the material to be welded are con- 

 nected in a simple electric circuit and an arc is struck by bringing the electrode in con- 

 tact with the work at the point where the weld is to be made, then withdrawing it 

 slightly to obtain the desired length of arc. 



The two principal methods of applying this process are by means of: — (a) the 

 carbon arc; (b) the metallic arc. 



In the former a carbon electrode is used and the heat from the arc produced 

 brings the metal to a fusion heat. When welding with the carbon arc additional 

 metal is introduced into the arc and fused into and with the parent metal at the 

 joint. Its main field of application, however, is in rough cutting in foundries and 

 steel mills and for the repair or building up of imperfect castings. 



It has not been advocated for use in ship construction generally and there are 

 a number of reasons which make it doubtful whether it ever will be. 



In metallic arc welding a metal electrode is used of approximately similar ma- 

 terial to that being welded, the electrode itself is fused by the arc, and molten par- 

 ticles are carried over the arc into the fused portion of the parent metal, thus grad- 

 ually building up the joint. 



The actual operation, however, is not quite so simple as this sounds, for there are 

 a large number of variables, any one or any combination of which may affect the 

 efficiency of the result to a greater or less extent. 



These variables may be enumerated as follows: — 



1. The type of electrode^ i. e., bare metal or covered. 



2. Chemical composition of the electrode. 



3. Chemical composition of metal being welded. 



4. Size of electrode. 



