Improvements in Electrically Heated Ovens.
Year: 1891

page_text: Date of Application, 11th Aug., 1891-Accepted, 14th Nov., 1891
COMPLETE SPECIFICATION.
Improvements in Electrically Heated Ovens.
I, WILLIS MITCHELL, of Malden, in the County of Middlesex, and State of
Massachusetts, United States of North America, Electrician, do hereby declare
the nature of my invention for IMPROVEMENTS IN ELECTRICALLY HEATED
OVENS to be particularly described and ascertained in and by the following
statement:-
This invention has for its objects. to increase the efficiency of electrically heated
ovens and to protect and insulate the heating wires more effectually and easily than
hitherto. For these purposes I construct the bottom, top and sides of my oven, or
some if not all, of these with successive layers of asbestos or other insulating
material, the heating wires being wound around the interior space of said oven in
successive coils or layers which are interposed between said layers and practically
embedded therein. The winding may be either vertical or horizontal and the wire
in each spiral of each layer is so far separated from every other spiral of every
layer as to give the best heating effect.
In the accompanying drawings,
Figure 1 represents a side elevation partly broken away an oven embodying my
invention, the said oven having vertical layers as above stated at its ends and sides
and the wire being wound horizontally.
Figure 2 represents a horizontal longitudinal section of the same and
Figure 3 represents a perspective view with one side removed of, an oven
having its top, bottom and sides only provided with such layers, those of the top
and bottom being of course horizontal, and the wire being wound vertically, as
stated.
A designates the metallic outer casing or outer walls of the oven. As shown in
Figure 1, parallel vertical layers B of asbestos or other insulating material are
arranged within the sides and ends of this casing their number depending on the
space conveniently available and the amount of wire to be used. The inner face
of the innermost layer is covered by a thin lining C of sheet metal.
D designates the heating wire, which forms, part of an electric circuit. In
producing or preparing the oven, I place the first layer of asbestos against the
outside of the inner lining C. I then wind the wire D nearly horizontally thereon,
the successive wire coils or spirals being considerably separated. In practice I find
it best to have the wire everywhere in a space about ten or twelve times its own
width. I then apply the second wire of asbestos plates or layers and wind similarly
thereon. This is continued until the coils of wire are sufficient to effect my purpose.
I then apply the outer layer of asbestos and the external casing and the oven is
complete.
As shown in Figure 3, the layers B may be omitted at the ends of the oven and
supplied at the top and bottom thereof instead. The arrangement of the insulating
layers of the oven will then be partly vertical and partly horizontal, those of the
top and bottom taking the latter position. The putting together and winding are
as before, except that the wire is wound approximately in vertical planes around.
the top, bottom and sides of the oven instead of being wound approximately in
norizontal planes, as before, around the sides and ends.
Of course the winding may be about the top, bottom and ends equally well, the
layers being supplied to the ends and omitted from the sides, everything being in
other respects as shown in Figure 2. It is not deemed necessary to illustrate
this, it being at most a trivial change of arrangement, and not amounting to a
modification.
Whichever way wound the coils or spirals of each wire layer are practically

page_text: embedded in the softer insulating material as they are drawn rather tightly on the
layer of asbestos within them, and the next outer layer is pressed on their exterior
either by the drawing of the outer wire layers or in the case of the outermost layer
by the weight and tight fitting of the outer casing A itself. This tight fitting is
not necessary however. Either with or without it the asbestos, or other soft
material substituted therefor, will hold each spiral of wire in its appropriate place
and thoroughly insulate it. The oven will be as easily taken apart as put together;
and is very simple, efficient and compact.
I do not depend largely on the heat of resistance, as when the wires become
red hot the meat or other article which is undergoing baking will be overcooked
on the outside and left raw within. Such at least is the tendency of heating by
this method. But when wound as I have stated and at intervals such as described,
the current through the wire produces an equable cooking effect throughout the mass
of meat, the heat being apparently greatest at the centre. I am not able to explain
the hidden causes of this action; but the fact is continually demonstrated.
In the outer casing A, I form by preference an air space a dividing it into a
double wall. But this is not indispensable.
Of course the construction hereinbefore described may be applied to general
heating as well as cooking uses.
Having thus described my invention, what I claim as new and desire to secure
by Letters Patent is :-
First An oven or heater provided with successive layers of non-conducting
material and a wire which forms part of an electric circuit and is wound in
successive layers between said non-conducting layers for the purpose set forth.
Second An oven or heater provided with successive layers of non-conducting
material and a wire which forms part of an electric circuit and is wound between
said layers and embedded therein, substantially as set forth.
Third An oven or heater provided with successive layers of asbestos or other
non-conducting material arranged about it and a wire forming part of an electric
circuit and wound between said non-conducting layers about said oven so as to form
successive layers of wire having each coil or spiral insulated and separated by the
material in which it is embedded, substantially as set forth.
Fourth An oven or heater provided with successive layers of wire surrounding
it and forming part of an electric circuit, said wire being coiled in spirals each of
which is sufficiently separated from all the others to heat the interior of the oven
without depending on the heat produced by the resistance of the wire, substantially
as forth.
Fifth The combination of a wire wound spirally in layers and forming part of
an electric circuit with layers of non-conducting material interposed between said
layers of wire and holding the spirals thereof in place an outer casing A and an
interior lining C, the whole constituting an electrically heated oven, substantially
as set forth.
Dotor the 4th day of [11]v 891
Lotod thre Latch day of July 891
WILLIS MITCHELL.
In presence of
ROBERT W. SCOTT.
P. R. TRIPp.