def scalar(magnitude = value)
      if special?
        unit.scalar(magnitude)
      else
        Number.rationalize(value) * Number.rationalize(unit.scalar)
      end
    end
def scalar(magnitude = value)
      if special?
        unit.scalar(magnitude)
      else
        Number.rationalize(value) * Number.rationalize(unit.scalar)
      end
    end
Get a scalar value for this scale.
 @param magnitude [Numeric] An optional magnitude on this scale.
 @return [Numeric] A scalar value on a linear scale
 @api public