The alternating current is employed. A special dynamo is sometimes used
to produce it. This dynamo has two windings on the armature. One is of
fine wire and is in series with the field magnets and excites them. The
other is of copper bars, and connects with the welding apparatus, giving
a current of high intensity but actuated by low potential.
Where the special dynamo is not used, an induction coil or transformer
is used. The primary includes a large number of convolutions of
relatively fine wire; the secondary may only be one turn of a large
copper bar.
The cut shows in diagram an electric welding coil. P is the primary coil
of a number of turns of wire; S S is the secondary, a single copper bar
bent into an almost complete circle. It terminates in clamps D D for
holding the bars to be welded. B C, B' C are the bars to be welded. They
are pressed together by the screw J. The large coil I of iron wire
surrounding the coils represents the iron core.
The real apparatus as at present constructed involves many
modifications. The diagram only illustrates the principle of the
apparatus.
In welding by the voltaic arc the place to be heated is made an
electrode of an arc by connection with one terminal of an electric
circuit. A carbon is connected to the other terminal. An arc is started
by touching and withdrawal of the carbon. The heat may be used for
welding, soldering, brazing, or even for perforating or dividing metal
sheets.
Welding Transformer.
The induction coil or transformer used in electric welding. For its
general principles of construction, see Welding, Electric.
Wheatstone's Bridge.
A system of connections applied to parallel circuits, including
resistance coils for the purpose of measuring an unknown resistance. A
single current is made to pass from A through two parallel connected
branches, joining together again at C. A cross connection B D has a
galvanometer or other current indicator in circuit. In any conductor
through which a current is passing, the fall of potential at given
points is proportional to the resistance between such points. Referring
to the diagram a given fall of potential exists between A and C. The
fall between A and B is to the fall between A and C as the resistance r
between A and B is to the resistance r + r' between A and C. The same
applies to the other branch, with the substitution of the resistances s
and S' and the point D for r r' and B. Therefore, if this proportion
holds, r : r' : : s : S'. No current will go through B D , and the
galvanometer will be unaffected. Assume s' to be of unknown resistance,
the above proportion will give it, if r, r' and s are known, or if the
ratio of r to r' and the absolute value of s is known.