976,411. Spectral analysis. FEEDBACK CONTROLS Inc. Feb. 22, 1962 [Feb. 24, 1961], No. 6859/62. Heading G1A. [Also in Division G4] In spectrochemical analysis apparatus in which a detector responds to spectral line radiation corresponding to an element whose concentration is to be determined, the detector output is employed to actuate a servo system to position the tapping of a potentiometer which is given a non- linear characteristic so as to compensate for the non-linear relationship which exists between the detector output and element concentration, the potential from the potentiometer tapping thereby providing a direct accurate indication of the actual concentration. The potentiometer Fig. 4 comprises a main voltage dividing element comprised of a plurality of fixed value segments 102 and a like plurality of auxiliary potentiometer elements 116 the tapping points of which are connected to the tapping points 106 of the main divider and which are initially preset so as to provide a series of step approximations to the non-linear relationship between the detector output and actual concentration. In a detailed construction, Fig. 3 (not shown), intended for four different concentration measurements, four complete sets of auxiliary potentiometers are provided which may be brought into use by a multi-bank four-way switch, and which are preset so as to provide four different non-linear characteristics. To allow for changes (due to spectro-chemical causes) which do not effect the shape of the non-linear curve but merely the slope and position with respect to the co-ordinate axis of the system, two potentiometers 120, 122 are provided which permit adjustment of the voltage applied to the main voltage divider 102 independent of the auxiliary potentiometer element 116. The potentiometers 120, 122 adjust the voltages applied to the top and bottom of the divider 102 corresponding to the high and low concentrations and thus control the slope and position of the non- linear curve. Fig. 1 illustrates a first embodiment in which light emanating from a unit 10 for arcing a specimen to be analysed is dispersed by a prism or grating 18 into its spectral line components which impinge on a series of exit slits 2 behind which are located photo-tubes 22. Although only four tubes are shown, more may be employed. Two further photo-tubes 36 and 37 are provided, the first being aligned to sense a principal spectral component and being employed to control the duration of arcing (see below), and the second providing a monitoring signal for meter 34. The output from photo-tubes 22 are integrated during the arcing period and are then applied via a selector switch 50, units 52, 90, 92 (see below), amplifier 94 and servo motor 96 to control the setting of the tapping 98 of a potentiometer 100 which is given a non-linear characteristic as described above. The potential from tapping 98 is applied via an amplifier 130 to control the servo motor 132 of a known form of decimal digitizer 134 which feeds its output via a storage unit 138 to a digital display unit 140, typewriter 142 and card punch unit 144. Selector switch 50 is ganged to potentiometer 100 so as to cause an appropriate non-linear characteristic to be introduced for each spectral line measured. Selector switch 50 also connects to photo-tube 36 for use during timing the duration of arcing. Unit 52 is an analogue-to-digital converter whose pulse output is counted in unit 90 and employed to terminate the arc when a preset count is reached. Unit 92 is a digital-to-analogue converter which produces a D.C. voltage to operate servo system 94, 96 during measurements. A suitable circuit for unit 52 is described with reference to Fig. 2 (not shown). In a second embodiment, Fig. 5 (not shown), only two photo-tubes are employed, one being permanently positioned on a standard spectral line and the other being driven by a motor screw drive to measure sequentially all spectral lines of interest. At each measurement, the ratio of the outputs of the two tubes is produced and this ratio signal is then employed to position the tapping of a non-linear potentiometer arranged as described above. A third embodiment, Fig. 6 (not shown), comprises an X-ray analysis system in which the spectrum of X-rays given off by a sample irradiated by X-rays is sensed by an array of detectors, e.g. Geiger tubes or scintillation counters, and the resulting outputs employed to control the non-linear potentiometer.