used for extension of wires not shown of the temperature sensors 26 to connect with a monitoring computer not shown the driving device 40 in this preferred embodiment is a step motor although it can be easily apprehended by those skilled in the art that the driving device 40 can also be a pneumatic cylinder or a hydraulic cylinder the driving device 40 is installed on the second plate 16 of the supporting frame 10 the driving device 40 is fixed to the second plate 16 above the movable portion 30 a shaft not labeled of the driving device 40 extends through the second plate 16 of the supporting frame 10 the shaft has a threaded end not shown threadedly engaging with a bolt 42 secured to a board 34 of the movable portion 30 the board 34 is fastened to the movable portion 30 when the shaft rotates the bolt 42 with the board 34 and the movable portion 30 is moved upwardly or downwardly two through apertures not labeled are defined in the board 34 of the movable portion 30 for extension of wires not labeled of the temperature sensors 36 to connect with the monitoring computer in use the driving device 40 drives the movable portion 30 to make accurate linear movement relative to the immovable portion 20 for example the movable portion 30 is driven to depart a certain distance such as 5 millimeters from the immovable portion 20 to.