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: 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 , , In this group the first place priority rule is applied, i.e. at each hierarchical level classification is made in the first appropriate place.