1.61 quantity of dimension one dimensionless quantity Quantity for which all the exponents of the factors corresponding to the base quantities in its quantity dimension are zero. Note 1: The term “dimensionless quantity” is commonly used and is kept here for historical reasons. It stems from the fact that all exponents are zero in the symbolic representation of the dimension for such quantities. The term “quantity of dimension one” reflects the convention in which the symbolic representation of the dimension for such quantities is the symbol 1 (see ref. 31). Note 2: The measurement units and values of quantities of dimension one are numbers, but the quantities convey more information than a number. Note 3: Some quantities of dimension one are defined as a ratio of two quantities of the same kind. Examples: Plane angle, solid angle, refractive index, relative permeability, mass fraction, friction factor, Mach number. Note 4: Numbers of entities are quantities of dimension one. Examples: Number of turns in a coil, number of molecules in a given sample, degeneracy of the energy levels of a quantum system. Source: [VIM 1.8].
 1.61 quantity of dimension one dimensionless quantity Quantity for which all the exponents of the factors corresponding to the base quantities in its quantity dimension are zero. Note 1: The term “dimensionless quantity” is commonly used and is kept here for historical reasons. It stems from the fact that all exponents are zero in the symbolic representation of the dimension for such quantities. The term “quantity of dimension one” reflects the convention in which the symbolic representation of the dimension for such quantities is the symbol 1 (see ref. 31). Note 2: The measurement units and values of quantities of dimension one are numbers, but the quantities convey more information than a number. Note 3: Some quantities of dimension one are defined as a ratio of two quantities of the same kind. Examples: Plane angle, solid angle, refractive index, relative permeability, mass fraction, friction factor, Mach number. Note 4: Numbers of entities are quantities of dimension one. Examples: Number of turns in a coil, number of molecules in a given sample, degeneracy of the energy levels of a quantum system. Source: [VIM 1.8].

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