In a popcorn cooking machine of a known type, an electric heating element is mounted on a container and a cooking temperature control thermostat is connected in series with the heating element to control the cooking temperature. The cooking temperature control thermostat is usually mounted on the outside of the cooking container and typically has a rotatable knob or other adjustable control for adjusting the cooking temperature which may be on the order of from 400.degree. to 500.degree. F.
Such machines usually operate quite satisfactorily for substantial periods of time and although problems have developed, they have not been recognized and dealt with, apparently because they usually develop only after relatively long periods of use. One of such problems is that the cooking temperature control thermostat may fail to open when the operating temperature exceeds the desired operating temperature. When this happens, the temperature may rise quite rapidly and if, without realizing that the cooking temperature control thermostat has failed, the operator places oil in the container, the oil will not only burn but may splatter to burn the operator. In a typical machine, the temperature will rise at a rate of about 50.degree. F. per minute from a normal operating temperature of 475.degree. F. to reach a temperature of 650.degree. F. in a little over three minutes at which temperature the oil will burn. If a longer time has elapsed, the temperature may be over 900.degree. F. and the reaction is almost explosive.
Such failures of the cooking temperature control thermostats are very likely to occur after prolonged periods of use because such thermostats typically comprise a bimetal element or the like which is effective to move two contacts apart to break the circuit and allow the heating element to cool, the contacts being then re-engaged to again complete the circuit. With the repeated cycling movements produced in long periods of use, a possible malfunction of the thermostat is likely and cannot be readily avoided by improvements in design.
The prior art also has deficiencies in that thermostats which might be effectively used for protecting against the aforementioned problems have not been available. The provision of a meltable fuse element for opening the circuit when a high temperature is reached is a possibility but is not a practical solution. Mechanical forces applied to the cooking container during normal operation can cause a fuse to be broken before the melting temperature thereof is reached and the replacement of a fuse can be a frustrating problem to the operator of a machine especially where the fuse is of a special type which is not generally available.
Thermostats which include bimetal disc elements or the like have had constructions such as not to be suitable for protecting against the aforementioned conditions. For example, one problem is that they have not been operative to open a circuit at a relatively high temperature without having the undesirable feature of being operative to reset themselves at a lower but still elevated temperature.
Thermostats of the type having elongated connections between a temperature sensing point and a point where electrical contacts are located and operated are not at all suitable for use in popcorn popping machines or the like.
As a result of the failure to recognize the problems involved and as a result of the inadequacies of the prior art devices, there has been no solution to the aforementioned dangerous problems which can result from failures of the cooking temperature control thermostats of popcorn popping machines or the like.