TO SHIP CONSTRUCTION. 207 



in Fig. 2, Plate 132. At the time of writing, the makers of this machine and others 

 are engaged in the development of portable automatic welders, about which we shall 

 no doubt hear more in the near future. 



8. The preparation and building up of the joint have considerable bearing not 

 only on the ultimate strength of the weld itself, but on the time and cost of mak- 

 ing it. 



Correct preparation of the joint for welding is just as important, if not more 

 so, as in the case of a riveted joint. The time and cost of welding depend largely 

 on the amount of electrode material deposited, and therefore the joint should be pre- 

 pared so as to require the minimum amount of building up consistent with the neces- 

 sary strength and accessibility. Plates 133 and 134 show a number of typical 

 welded joints which may be met with in ship construction with a brief description of 

 the method of preparing and welding as recommended by the Welding Committee. 



In large plates where a long joint, such as a seam, has to be welded, the con- 

 traction set up in cooling tends to draw the joint together, and, unless proper pre- 

 cautions are taken, the plates will tend to fold over one another at the end away 

 from that being welded. 



Two methods to counteract this are generally adopted. In the "non-rigid" 

 method the two plates, instead of being placed parallel with the joint, are diverged 

 to the extent of about 1/8 inch to the lineal foot of weld; as the welding proceeds 

 from one end, the plates gradually close together without setting up any great initial 

 internal stresses. This method obviously introduces a number of practical difficul- 

 ties if applied to the main construction of a vessel. 



In the "rigid" method the joint is tack welded at intervals along its length 

 and the welding done, not continuously from end to end, but in separate stages so as 

 to distribute the heat, as much as possible, uniformly along the joint. 



An interesting method of building up a "rigid" joint in order to reduce the in- 

 ternal contraction stresses is indicated in Plate 135, where it will be observed that 

 each side of the joint is separately built up, the deposited material from the elec- 

 trode being entirely free to contract on one side, leaving only the "closing in" run 

 of metal which is small and which is bonded on each side of the joint. 



WELDING APPARATUS. 



The types of apparatus manufactured for electric arc welding may be divided 

 into three main classes : 



I. Motor generator sets operating from A. C. or D. C. mains and designed to 

 generate a suitable voltage. These machines are generally supplied with controls 

 which automatically adjust the current to the length of arc operating. 



Examples of these machines are illustrated in Figs, i and 2, Plate 136. 



The first is a 4-arc D. C. motor generator set of 1 5-kilowatt capacity manufac- 

 tured by the General Electric Company. With this machine four balancer sets and 

 control panels are supplied, which it is claimed give thorough protection to the gen- 



