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 This subclass does not cover features of general applicability to regulating systems, e.g. anti-hunting arrangements, which are covered by subclass . In this subclass, the following term is used with the meaning indicated: "systems" includes self-contained devices such as speed governors, pressure regulators. Control systems specially adapted for particular apparatus, machines or processes are classified in the subclasses for the apparatus, machines or processes, provided that there is specific provision for control or regulation relevant to the special adaptation, either at a detailed level (e.g. : "for regulating temperature in bakers' ovens") or at a general level (e.g. : "for automatic control of welding parameters in arc welding"). Otherwise, classification is made in the most appropriate place in this subclass. The following are lists of places where there is specific provision of the kind referred to above. Where such provision is at a detailed level, the places have been grouped according to the main groups of this subclass. Where the provision is at a general level (e.g. of a kind appropriate to more than one of the main groups specified in the lists, or to main groups or ), the places are listed under the title "General References". Places related to Agricultural machines or implements Toy vehicles Air-cushion vehicles Road vehicle path control Steering controls of motor vehicles or trailers, i.e. means for initiating a change of direction Arrangements for automatically controlling the steering depending on driving conditions Chassis of endless-tracked vehicles Marine steering; control of waterborne vessels Controlling aircraft Controlling attitude or direction of aircraft ejector seats Cosmonautic vehicles Self-propelled missiles Guided missiles Marine torpedoes Places related to Footwear manufacture Tool carriers in forging or pressing Pattern-controlled boring or drilling tools ,, Planing or slotting machines controlled by copying device Electrode to workpiece spacing in electric discharge and electrochemical machining Workpiece in laser welding or cutting Workpiece in welding Molten metal in welding Spindles in machine tools , Tool or work position in machine tools Tools controlled by pattern or master model Grinding controlled by patterns, drawings, magnetic tape or the like Starting position in grinding Actuating members in presses Chassis of tracked vehicles Web-advancing mechanisms Dippers or buckets in dredgers Fluid-pressure servomotors with follow-up action Tracking of solar heat collectors Photomechanical production of patterned or textured surfaces Rotating heads in information storage systems Movement of control elements in nuclear reactors Places related to Tobacco cutting Thickness of coating of fluent material on surface Thickness, width, diameter or other transverse dimensions of the products of metal-rolling mills Dimension of glass ribbon Thickness of layer in paper making Places related to Air in hair drying helmets Flow of media to the human body Gases or vapour in electrostatic separators Fluent material in coating devices Dispensing beverages on draught Transferring liquids Gas purifiers Flushing boreholes Obtaining liquids from wells Flow in non-positive-displacement machines or systems Lubrication arrangements Coolant flow in cooling devices , Gas-turbine working fluid Throttle passages in pipes Air-flow or supply of heating or cooling fluids in air treatment arrangements Air or gas flow in dryers Continuous flow weighing apparatus Coolant in nuclear power plant Places related to Liquid level in sedimentation arrangements Ink level in printing, manifolding or duplicating arrangements Feed water for boilers , Liquid pool electrodes in electric discharge tubes or lamps Places related to Density in sedimentation arrangements Mixers Abrasive blasts Mixtures of clays or cements Bulk material conveyers Flow ratio in jet-propulsion plants Places related to Drum speed in metal drawing Cutting velocity of tool or work Ram speed in presses Setting or limiting speed of vehicles Electrically-propelled vehicles Road vehicle cruise control Cruising speed of aircraft Feed rate in manufacture of artificial filaments, threads, fibres, bristles or ribbons Carding machines Warping, beaming or leasing machines Cyclically varying speed of looms Speed of fluid carrier in chemical analysis Filamentary or web record carriers or heads for such carriers in information storage systems Non-filamentary, non-web record carriers, or heads for such carriers in information storage systems Places related to Portable percussive tools Ram pressure in presses Tension in filamentary material Tension in webs, tapes, filamentary material Rope, cable or chain tension Tension in looms Tension in sewing machines Pressure in paper-making machines Drying fabrics Pressure in dryers Record carrier tension in information storage arrangements Places related to Tyre pressure Air within diving suit Aircraft air-pressure Bulk material conveyers Manufacture of artificial filaments, threads, fibres, bristles or ribbons Flushing boreholes Lubrication arrangements Pressure of fluid carrier in chemical analysis Pressure in electric discharge tubes or lamps Pressure in electric incandescent lamps Places related to Portable percussion tools Jigging conveyers Places related to Density in sedimentation arrangements Treating gases or vapours Composition of fluid carrier in chemical analysis Places related to Watering gardens, fields, sports grounds or the like Poultry incubators Tobacco products Air conditioning Dryers Places related to Bakers' ovens Hair curlers Metal extruding Tyre temperature Cosmonautic vehicles , Float baths in glass making Manufacture of artificial filaments, threads, fibres, bristles or ribbons Knitting machines Hand irons Paper-making machines Lubricant in lubrication arrangements Arrangements for supplying oil or unspecified lubricant from a reservoir Steam superheat Dryers Temperature of fluid carrier in chemical analysis Electric storage cells , , Dielectric, induction or microwave heating Anode of X-ray tube Places related to Photographic composing machines ,, Lasers and other light sources General references Combines Milking machines Welding parameters Copying , Grinding or polishing Abrasive blasts Dispensing beverages on draught Combustion apparatus in which combustion takes place in a fluidised bed of fuel or other particles Electrographic, electrophotographic or magnetographic processes Dynamo-electric motors or generators CONTROL OF: SPEED OR ACCELERATION; FORCE; PRESSURE; POWER; MECHANICAL OSCILLATIONS ; ; ; ; CONTROL OF: FLOW; LEVEL; RATIO ; ; CONTROL OF: TEMPERATURE; HUMIDITY; VISCOSITY; CHEMICAL OR PHYSICO-CHEMICAL VARIABLES; LIGHT INTENSITY ; ; ; ; CONTROL OF: POSITION, DIRECTION, DIMENSIONS SIMULTANEOUS CONTROL OF TWO OR MORE VARIABLES , SUBJECT MATTER NOT PROVIDED FOR IN OTHER GROUPS OF THIS SUBCLASS This subclass covers: systems only; use of hydraulic, pneumatic, mechanical, and electrical motors for varying electric characteristics of devices which restore the quantity regulated; the combination of static converters and current or voltage regulators, if the essential characteristic resides in the combination. This subclass does not cover elements per se, which are covered by the relevant subclasses. This subclass covers: members of general applicability for mechanical control; mechanical systems for moving members to one or more definite settings. Systems peculiar to the control of particular machines or apparatus provided for in a single other class are classified in the relevant class for such machines or apparatus, for example: Controls for adjusting operating tables Controls for adjusting operating chairs Accessories for games using an electronically generated display Manipulators, e.g. controls therefor Arrangement or mounting of propulsion-unit control devices in vehicles Vehicle brake-action initiating means Adaptations of control devices for movable vehicle cabs Cycle-steering devices Rider-operated controls specially adapted for cycles Brake-actuating mechanisms specially adapted for cycles Marine steering initiating means Controls for elevators Control systems or devices for cranes Arrangements of handles or pedals for crane operation Control devices for dredging or soil shifting machines Adjustable cranks or eccentrics Automatic clutches , Controls for valves Safety devices acting in conjunction with the control or operation of a machine Stopping machines on occurrence of dangerous conditions therein Micromanipulators structurally combined with microscopes Driving mechanisms in clocks or watches Digital computers in which all the computation is effected mechanically Manual computer input arrangements Converting a pattern of mechanical parameters into electric signals Displacement of solid control elements in nuclear reactors Mechanisms for operating switch contacts Mechanical control of resonant circuits. MANUALLY-ACTUATED CONTROL MECHANISMS, ONE OR MORE CONTROLLING MEMBERS ACTUATING ONE OR MORE CONTROLLED MEMBERS , , , AUTOMATIC MOVEMENT-INITIATING DEVICES; TRIP MECHANISMS ; SERVO-MECHANISMS PROGRAMME-CONTROL DEVICES LOCKING MEANS, LIMITING MEANS; POSITIONING MEANS ; COMPONENT PARTS , ,