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. Some appear in the notes of several of the classes in this section, see in particular the definition of "measuring" in class G01. Others appear in paragraph 187 of the Guide to the IPC, see in particular the definitions of "control" and "regulation".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 covered by class G09, for enabling the result of a measurement to be known covered by class G01, for signalling the information to a distant point or for giving information which has been signalled from a distant point covered by class G08. 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 covered by subclass G01L or about some other condition linked to the pressure covered by another subclass of class G01, e.g. G01K for temperature, to make a record of the pressure or of its occurrence covered by subclass G07C, to give an alarm covered by subclass G08B, or to control another apparatus covered by class G05.
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.