The one great advantage of the oxy-acetylene method in comparison with
other methods of welding is the fact that in some cases of very thin sheet,
the weld can be made somewhat sooner than is possible otherwise. With metal
of 18 gauge or thicker, this advantage is eliminated. In cutting steel, the
oxy-acetylene torch is superior to almost any other possible method.
_Arc Welding Machines._--A consideration of the function and purpose
of the various types of arc welding machines shows that the only reason for
the use of any machine is either for conversion of the current from
alternating to direct, or, if the current is already direct, then the
saving in the application of this current in the arc.
It is practically out of the question to apply an alternating current arc
to welding for the reason that in any arc practically all the heat is
liberated at the positive electrode, which means that, in alternating
current, half the heat is liberated at each electrode as the current
changes its direction of flow or alternates. Another disadvantage of the
alternating arc is that it is difficult of control and application.
In all arc welding by the use of the carbon arc, the positive electrode is
made the piece to be welded, while in welding with metallic electrodes this
may be either the piece to be welded of the rod that is used as a filler.
The voltage across the arc is a variable quantity, depending on the length
of the flame, its temperature and the gases liberated in the arc.
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