Abstract: A sheathed heater wherein before the sheathed heater is assembled, an oxide film is formed on the inner peripheral surface of a metal pipe, and/or the outer surfaces of an internal heater and lead pins thereof which are inserted into the metal pipe by heating them in the presence of oxygen or by applying chemicals thereto, and a heat resistent insulation material member is filled in a space formed between the metal pipe and the internal heater. The color of the oxide film thus formed is light brown, brown or black.

Abstract: A position feedback system capable of effectively preventing an NC apparatus from failing in a position signal reading operation due to mechanical vibration of a machine tool or the like, including an up-down counter for counting up or down phase signals supplied from a machine position encoder, a comparator for comparing the counted value with a reference value, and a pulse generator for supplying an up- or down-counting pulse having a time period larger than a minimum time period necessary for the reading operation of the NC apparatus to the comparator to initiate a comparison operation regarding the counter contents which generally reflects the number of phase signals, since spurious noise signals have a tendency to cancel each other out. Based on the comparison, a counting pulse is fed to the NC apparatus and a pulse is applied to the counter to cancel one counted value. The operation is carried out until the counted value becomes the same as the reference value.