In this section, the following term is used with the meaning indicated: "variable" (as a noun) means a feature or property (e.g., a dimension, a physical condition such as temperature, a quality such as density or colour) which, in respect of a particular entity (e.g., an object, a quantity of a substance, a beam of light) and at a particular instant, is capable of being measured; the variable may change, so that its numerical expression may assume different values at different times, in different conditions or in individual cases, but may be constant in respect of a particular entity in certain conditions or for practical purposes (e.g., the length of a bar may be regarded as constant for many purposes). Attention is drawn to the definitions of terms or expressions used, appearing in the notes of several of the classes in this section, in particular those of "measuring" in class and "control" and "regulation" in class . Classification in this section may present more difficulty than in other sections, because the distinction between different fields of use rests to a considerable extent on differences in the intention of the user rather than on any constructional differences or differences in the manner of use, and because the subjects dealt with are often in effect systems or combinations, which have features or parts in common, rather than "things", which are readily distinguishable as a whole. For example, information (e.g., a set of figures) may be displayed for the purpose of education or advertising (), for enabling the result of a measurement to be known (), for signalling the information to a distant point or for giving information which has been signalled from a distant point (). The words used to describe the purpose depend on features that may be irrelevant to the form of the apparatus concerned, for example, such features as the desired effect on the person who sees the display, or whether the display is controlled from a remote point. Again, a device which responds to some change in a condition, e.g., in the pressure of a fluid, may be used, without modification of the device itself, to give information about the pressure () or about some other condition linked to the pressure (another subclass of class , e.g., for temperature), to make a record of the pressure or of its occurrence (), to give an alarm (), or to control another apparatus (). The classification scheme is intended to enable things of a similar nature (as indicated above) to be classified together. It is therefore particularly necessary for the real nature of any technical subject to be decided before it can be properly classified. This class covers methods, systems, and apparatus for controlling, in general. In this class, the following terms or expressions are used with the meanings indicated: "controlling" means influencing a variable in any way, e.g. changing its direction or its value (including changing it to or from zero), maintaining it constant, limiting its range of variation; "regulation" means maintaining a variable automatically at a desired value or within a desired range of values. The desired value or range may be fixed, or manually varied, or may vary with time according to a predetermined "programme" or according to variation of another variable. Regulation is a form of control; "automatic control" is often used in the art as a synonym for "regulation". Attention is drawn to the Notes following the title of section G, especially as regards the definition of the term "variable". This subclass covers features of control systems or elements for regulating specific variables, which are clearly more generally applicable. This subclass does not cover: systems for controlling or regulating non-electric variables in general, which are covered by subclass ; systems for regulating electric or magnetic variables in general, which are covered by subclass ; systems specially adapted for the control of particular machines or apparatus provided for in a single other subclass, which are classified in the relevant subclass for such machines or apparatus, provided that there is specific provision for control or regulation relevant to the special adaptation (see Note (5), below). Otherwise, classification is made in the most appropriate place in this subclass. In this subclass, the following terms or expressions are used with the meanings indicated: "automatic controller" means a system, circuit, or device in which a signal from the detecting element is compared with a signal representing the desired value and which operates in such a way as to reduce the deviation. The automatic controller generally does not include the sensitive element, i.e. that element which measures the value of the condition to be corrected, or the correcting element, i.e. that element which adjusts the condition to be corrected; "electric" includes "electromechanical", "electrohydraulic" or "electropneumatic". In this subclass, details of specific control systems are classified in the group relevant to the system, if not otherwise provided for. This Note lists places in the IPC where there is specific provision of the kind referred to in Note (2)(c), above; where such provision is at a general level, the places are listed under the heading "General references"; where the provision is related to programme control, the places are listed under the heading "Places related to group ". General references Spreading or positioning of drawn nets for fishing , Adjustable operating tables, operating chairs, or dental chairs Distillation , , , Filtration Separation of gases or vapours by gas-analysis apparatus Separation using semi-permeable membranes Feed or outlet in chemical or physical processes Oxygen content in oxidation gas for regeneration or reactivation of catalysts Ion-exchange processes Delivery in spraying systems , Metal-rolling mills Positioning tool carriers for forging, pressing or hammering Continuous casting of metals Centrifugal casting of metals Pressure or injection die casting of metals Pressure or vacuum casting of metals Casting of metals in general Tool or work positioning for boring or drilling Machines for shearing or similar cutting stock travelling otherwise than in the direction of the cut Driving or feeding mechanisms of machine tools Feed movement, cutting velocity or position of machine tools Copying from a pattern or master model for machine tools Position of grinding tool or work Manipulators Position of cutters in cutting machines to Shaping techniques for plastic substances , Presses Composing machines Printing machines or presses Feeding sheets or webs in typewriters Apparatus or devices for manifolding, duplicating or printing for commercial purposes Addressing machines to Vehicle suspension to Vehicle brakes Machines for packaging Conveyers Sequence of drive operations for dredging or soil-shifting Earth drilling operations , Steam accumulators , , Steam engine plants Air intakes for gas-turbine or jet-propulsion plants Gas-turbine plants; Fuel supply in air-breathing jet-propulsion plants Combustion engines , Jet pipes or nozzles in jet-propulsion plants to Jet-propulsion plants , , Positive-displacement machines , Non-positive-displacement pumps, pumping installations, or systems , Clutches Suppression of vibrations using fluid means to Gearings Steam boilers Incineration of waste Combustion in combustion apparatus Combustion in open fires using solid fuel Solar heating Drying processes of solid materials or objects Steam or vapour condensers Heat-exchange apparatus with intermediate heat-transfer medium in closed tubes passing into or through conduit walls, in which the medium condenses and evaporates Heat-exchanges or heat-transfer apparatus in general Computers Traffic Indicating devices using static means to present variable information , Driving, starting or stopping of record carriers Nuclear reaction Nuclear power plant Electron-beam or ion-beam tubes used for localised treatment of objects Electric motors, generators, or dynamo-electric converters Places related to group (programme-control systems) Programmed medicine dispensers Disinfection or sterilising Heart pace-makers Steering-mechanisms for toy vehicles Centrifuges Thickness of work produced by metal-rolling mills Bending metal rods, profiles, or tubes , Boring or drilling machines Electrical discharge or electrochemical machining Assembling of parts to compose units Series of individual steps in grinding a workpiece Manipulators Presses Sequence of operations in printing machines or presses Feeding sheets or webs in typewriters Sequence of operations in apparatus or devices for manifolding, duplicating or printing for commercial purposes Selecting text or image to be printed in addressing machines Traction-motor speed of electrically-propelled vehicles Piling articles , Crane drives Dispensing, delivering, or transferring liquids , Sewing machines Embroidering machines Operations in washing machines , Combustion engines Supply of combustible mixture or its constituents to combustion engines Fluid-pressure actuator systems , Combustion in combustion apparatus Weighing apparatus , Electromechanical clocks or watches Mechanically operating digital computers Control units for electric digital data processing Peripheral devices for electric digital data processing Electrically operating digital computers Electrically or magnetically operating analogue computers , , Electrically-operated teaching apparatus or devices Electric switches Electron-beam or ion-beam tubes used for localised treatment of objects Electronic switching or gating , Selecting arrangements in electric communication technique CONTROL SYSTEMS Adaptive Controlled by computer Involving the use of models or simulators Controlled by programme Involving sampling Open-loop automatic control systems not otherwise provided for SYSTEM DETAILS Comparing elements Anti-hunting arrangements Internal feedback arrangements Obtaining smooth engagement or disengagement of automatic control Safety arrangements Automatic controllers TESTING, MONITORING SUBJECT MATTER NOT PROVIDED FOR IN OTHER GROUPS OF THIS SUBCLASS In this group, the measuring system for an axis is used to measure the displacement along that axis. This measurement is used as position-feedback in the servo-control system.