As is known, the resistance temperature coefficient of a resistive wire represents a function of its resistance variation with temperature variation, i.e. it reflects the stability of a resistance value with time for a specific resistive wire. The resistance value stability of a resistive wire is governed by the physical-chemical processes occurring in its material. One of the most important factors affecting these processes is the kind of thermal treatment to which the resistive wire is subjected. The properly chosen conditions of its thermal treatment tend to stabilize physical-chemical processes taking place in the wire. This provides a resistive wire with a small value of the resistance coefficient. Optimum thermal treatment conditions are determined by means of deriving a function of the resistance temperature coefficient versus variation of thermal treatment conditions. In practice, this function or characteristic is derived by thermally treating resistive wire specimens made from a single-smelting alloy under different conditions. The degree of accuracy of the function or characteristic obtained is largely dependent on the number of the tests performed as well as on the degree of the accuracy of setting of the temperature of thermal treatment. As experience shows, the resistive wire made from the same alloy but of a different smelting exhibits a different function or characteristic of the resistance temperature coefficient versus variation of thermal treatment conditions so that in order to derive this function or characteristic it is necessary to perform the tests again.
At present, to obtain function or characteristic of the resistance temperature coefficient of a particular resistive wire versus variation of conditions of its thermal treatment, known laboratory vacuum furnaces are utilized which are similar in construction to industrial furnaces employed for thermal treatment of metals. Resistive wire specimens are thermally treated in these furnaces under different temperature conditions within a temperature range in which the achievement of optimum thermal treatment conditions is expected.
The dependence of the resistance temperature coefficient on the variation of thermal treatment conditions is derived from the test results, and according to the dependence or relationship obtained optimum conditions are chosen.
The construction of vacuum furnaces intended for thermal treatment of metals does not enable thermal treatment of several resistive wire specimens to be performed simultaneously under different conditions.
From the foregoing it follows that the determination of optimum thermal treatment conditions by means of such furnaces is an extremely labour-consuming procedure.
Furthermore, the data thus obtained are not reliable inasmuch as the construction of the furnaces does not afford of a sufficient extent accurate setting to the desired temperature conditions.
The above stated essential disadvantages inherent in vacuum furnaces for thermal treatment of metals are partially eliminated by a gradient sublimator for crystallization (see a U.S. "Journal of Crystal Growth", 22, 1974, pp. 295-297), which can be employed to determine thermal treatment conditions for a specific resistive wire and, therefore, has been taken as a prior art prototype of the present invention. The device under consideration comprises a means for producing a temperature gradient in a crystal being examined, designed in the form of a hollow metallic cylinder with one end thereof carrying an electric heater and the other end carrying a cooler. Within the cylinder there is arranged a glass vacuum chamber having its base connected to a vacuum pump, and intended to contain therein the crystal being examined.
Upon turning on the heater and the cooler a temperature gradient is developed in the crystal under study. The temperature gradient in the crystal is caused by a radiant thermal energy being emanated by the inner surface of the hollow cylinder. Instead of the crystal under study, inside the vacuum chamber, there may be located a resistive wire specimen in which a temperature gradient also arises.
While the above device can provide simultaneous heating of a resistive wire specimen up to different temperatures along the length thereof, the linear character of lengthwise temperature gradient distribution is absent. This is accounted for by the fact that each portion over the length of the resistive wire specimen will be heated by the radiant thermal energy being emanated by various portions of the inner surface of the hollow cylinder. Moreover, the temperatures of the portions over the length of the specimen will not be stable with time, which is attributed to the influence of the environment on the means for producing a temperature gradient.
In view of the above, it becomes perfectly evident that with the prior art device in question it is impossible to find out a reliable dependence of the resistance temperature coefficient on the variation of thermal treatment temperatures. Hence, such a dependence does not allow one to ascertain optimum conditions under which the resistive wire is to be exposed to thermal treatment.
From the exemplary matter considered hereinabove it is obvious that the present state of the art does not include any special devices enabling expeditious and reliable determination of optimum resistive wire thermal treatment conditions.