The document describes a temperature controller or overheating safeguard device for electrical hot plates. It uses a ferromagnetic armature fixed to the hot plate that loses its magnetism at a specific Curie point temperature. A permanent magnet on a lever is also mounted on the hot plate and interacts with the armature to open or close an electric switch controlling current to the hot plate. When the temperature rises above the Curie point, the armature loses magnetism, causing the magnet to move away and open the switch to cut off current.

1. * GB780158 (A)
Description: GB780158 (A) ? 1957-07-31
Temperature controller or over-heating safeguard device for electrical hot
plates
Description of GB780158 (A)
PATENT SPECIFICATION
780,158 Date of Application and filing Complete Specification: Jan.
11, 1956.
No. 941/56, Application made in Sweden on Jan. 12, 1955.
Complete Specification Published: July 31, 1957.
Index at Acceptance:-Classes 38(5), B1R(12: 13P); and 39(3), H3C.
International Classification:-HOlh. H05b.
COMPLETE SPECIFICATION
Temperature controller or over-heating safeguard device for electrical
hot plates We, ALLMANNA SVENSKA ELEKTRISKA AKTIEBOLAGET, a Swedish
Company of VasterAs, Sweden, do hereby declare the invention, for
which we pray that a patent may be granted to us, and the method by
which it is to be performed, to be particularly described in and by
the following statement:-
The present invention relates to a temperature controller for, or a
device for safeguarding against over-heating of, a hot plate in an
electric cooker, which is based on the well-known fact that a
ferromagnetic substance loses most of its magnetic permeability at a
temperature which is characteristic for the substance and which is
known as the Curie-point.
According to the invention a temperature controller or safety cut-out
for a hot plate, comprising a movably arranged permanent magnet which
actuates a contact member of an electric switch, and a temperature
sensitive armature made of a soft ferromagnetic substance with a
suitable Curie-point, which armature cooperates with the said magnet
and at temperatures below the Curie-point forms a closed magnetic
circuit with the permanent magnet, is characterised in that the
armature is fixed on the hot plate in heat conducting contact with the
lower side of said plate, and in that the permanent magnet is arranged

2. on a lever, which by means of a spring or a pivot is pivotally mounted
on the hot plate,- movement of the magnet causing actuation of said
contact member to control the supply of current to the hot plate.
Preferably, the magnet is urged by gravity in the direction away from
the armature.
The device can be used either as a safeguard against over-heating,
which at an unallowable high temperature interrupts the current to the
plate, or as a temperature controller, to give the plate a constant
temperature. When employed as an over-heating safeguard the device
must be re-engaged by [Price 3s. 6d.] Prim 4 6d hand. When employed as
a temperature controller the device must by re-engaged automatically
when the temperature falls.
In this case the movement of the permanent magnet is limited in such a
way that, when the temperature falls, the recovering magnetic force
becomes greater than the force of gravity upon the magnet. The magnet
is then moved up to the armature and current is once again delivered
to the plate through the circuit breaker.
The temperature sensitive armature may consist of nickel or an alloy
of nickel and chromium and a suitable composition of the alloy for the
working temperature is used.
One form of a device according to the invention will now be described
by way of example with reference to the accompanying drawing, in which
Figure 1 shows a section of a hot plate with a temperature controller,
the magnetically operated switch being closed, and Figure 2 is a view
similar to Figure 1, the magnetically operated switch being open.
In the Figures, 1 is an electrically heated plate and 2 a heating
element. 3 is a temperature sensitive armature, arranged in heat
conducting contact with the lower side of the plate. A permanent
magnet 6 is arranged upon a lever 4. This lever also carries one of
the contacts 7 of a switch and is mounted upon a shaft 5 in the plate
1.
8 is a stop for limiting the movement of the lever 4.
When the temperature of the heating plate 1, and thus of the armature
3, is raised above the Curie-point of the latter, the magnetic
characteristics of the armature are changed, and the magnetising force
decreases. The magnet is then pulled down to the stop 8 by the force
of gravity and the switch is opened. When the temperature falls below
the said Curie-point the magnet is attracted and is moved up to the
armature and current is again delivered to the heating element.
-A ':
7: - - A slightly modified form of the device shown in Figures 1 and 2
may be employed as a safety cut-out. In this case the stop 8 would be
in such a position that, upon overheating of the hot plate, the magnet
6 would move sufficiently far from the armature 3 that the magnetic

3. forces would be unable to lift the magnet up to the armature when the
temperature fell below the Curie-point. A device of this nature would
be provided with a manually-operable re-setting means.
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* 5.8.23.4; 93p