where "ε"0 is the vacuum permittivity, a quantity with "dimension", namely (charge)2 (time)2 (mass)−1 (length)−3. Without "ε"0, the two sides would not have consistent dimensions in SI, whereas the quantity "ε"0 does not appear in Gaussian equations. This is an example of how some dimensional physical constants can be eliminated from the expressions of physical law simply by the judicious choice of units. In SI, 1/"ε"0, converts or scales flux density, D, to electric field, E (the latter has dimension of force per charge), while in "rationalized" Gaussian units, electric flux density is the same quantity as electric field strength in free space.