Patent GB-636544-A (GB)
Title: Improvements in or relating to optical pyrometers
IPC: G01J5/52
Filed: 1947-02-01
Abstract: 636,544. Pyrometers. BARBER, C. R. Feb. 1, 1947, No. 3136. [Class 97 (iii)] In a disappearing filament optical pyrometer the resistance of the lamp is measured as an indication of its temperature, the lamp forming part of a bridge circuit, the slider arm of which is ganged to the battery supply rheostat so that the bridge is maintained substantially in balance as the current through the lamp is increased. All the elements of the circuit, which is shown in Fig. 3, are contained in the pyrometer except the battery B and its leads. Fig. 1 shows the construe- ' tion of the pyrometer, which consists of a drum 1 having an objective 2 on one side and an eyepiece tube on the other. The optical system consists of a plano-convex objective lens adjustable by a pin sliding in a helical slot in a rotatable tube 38, stops 41, 48, 51 and 52 on erecting lens 47, a filter 50 and a Ramsden eyepiece. A neutral filter may be inserted at 56 and rotated to interrupt the light, if desired, to increase the range of the instrument. The logarithm of the transmission factor of this filter should be inversely proportional to wavelength. The filament is made flat and set with its width transverse to the optical axis to avoid end effects. " V " bends are made at its ends to maintain the filament straight when current is passed. The series battery rheostat 9 is mounted in the face of an insulating ring 5. A further insulating ring 6, mounted in a recess on the edge of ring 5, has the slider wire 10 mounted on its face. These two rings may be of moulded synthetic resin composition. An arm 13 of synthetic resin laminate rotated by a knurled knob 15 carries spring-loaded silverplated contacts 16 wiping the two resistors 9 and 10 and silver slip-rings 17. A felt rod dipped in clock oil may lubricate contacts 16. The contact pressure is adjusted by a nut 18. Arm 13 carries a pointer 20 co-operating with a scale 21 carried by sliding ring 6. Mounted on the back of plate 7 which supports ring 5 is a centre zero galvanometer 12 which may bear temperature calibrations on it and the pointer of which is visible through a window in the drum. A small part of the periphery of ring 6 is toothed and is rotatable by a knob 26 which is pressed inwardly to engage a pinion 24 with these teeth and to close the galvanometer circuit at contacts 27. A latch 28 holds this knob in. Sighting windows 32 and 34 are provided in the drum. A button 53 operates a switch for the lamp and moves a shutter 54 from the eyepiece. A trimmer rheostat X<SP>1</SP> is provided to alter the resistance in'the battery circuit for different values of battery voltage but correct matching between resistors 9 and 10 will be obtained only at the voltage for which the resistors are designed. In operation the instrument is sighted through windows 32 and 34, the filament then being viewed through the eyepiece and arm 13 rotated until it disappears. The position of pointer 20 on scale 21 may be read and added to the galvo reading. Alternatively knob 26 will move ring 6 independently of ring 5 so that the instrument may be brought to zero and the temperature read direct from scale 21. It is stated that for correct matching between resistors 10 and 9, resistor 9 must follow a parabolic law and is accordingly wound on a former of uniformly decreasing width. If a lamp of different characteristics is inserted a shunt may be placed across resistor 10. The Specification also details the method of calculating the various resistance values.