Melting Brass in a Rocking Electric Furnace Page: 69
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EXPERIMENTS WITH LARGE ROCKING FURNACE.
ture the are will run smoothly and quietly, even if the furnace is not
tight. With the furnace hot and tightly sealed, the are can not be
heard.
The presence of a nonconducting vapor, such as oil vapor, makes
the arc hard to hold. When oil is being driven off from an oily
charge, there is a well-defined point at which the arc suddenly
"catches" and begins to hold steadily, this being coincident with a
sudden drop in the volume of the oil flame from the open spout and
in the emission of soot.
Zinc vapor also has a marked effect on the are. Tif an alloy of
high zinc content is being melted and rocking is not started early in
the heat, there comes a time when the arc suddenly takes a higher
current and has to be lengthened in order to decrease the current
and to keep the power input normal. The natural tendency is to
draw the electrode out rapidly and too far, thus causing the arc to
break. If rocking is started, the are at once becomes normal, because
when the charge is stirred less zinc vapor is produced and the super-
heating of the surface layer is prevented. If the angle of rocking is
kept small, the phenomenon will be repeated at the point at which
the roof begins to reflect heat more rapidly than the metal can
absorb it, so that the surface again becomes superheated. Then, if
the full rocking angle is used, the heat in the roof is taken up by the
metal, stirring is again sufficient, and the are becomes steady. Still
later, in melting an alloy very high in zinc, the are again shows
signs of unsteadiness as the charge approaches the pouring tempera-
ture, because zinc vapor is then being given off from the thoroughly
stirred charge and not merely from a surface layer. The unsteadi-
ness increases with rising temperature. At about this time a tiny
zinc flame may break through the luting of the door or, if the elec-
trodes do not fit tightly into their holes, zinc flame may escape about
the electrodes. The flame may or may not appear, according to the
tightness of the electrodes and the door, but the behavior of the arc
is a good guide as to whether or not a charge high in zinc has reached
the proper pouring temperature.
For melting ordinary red brass the length of the are at the end of
the heat is normally about an inch, although it can be made longer
or shorter without greatly affecting the power input. For alloys
high in zinc the are draws out to a length of 2 or 3 inches at the end
of the heat and must be held at the proper length within more nar-
row limits. If the are is too short, too high power will be drawn,
and if the proper length is exceeded the arc breaks. On 220 volts
the are was drawn out at the end of the heat of yellow brass to more
than a foot, giving too high a power input at any length short of
that at which the are would break.69
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Gillett, H. W. & Rhoads, A. E. Melting Brass in a Rocking Electric Furnace, report, 1918; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc12367/m1/75/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.