def get_description
      similar = @@dimensions.find { |quantity| quantity == self }
      self.physical_quantity = similar.nil? ? nil : similar.physical_quantity
    end
def get_description
      similar = @@dimensions.find { |quantity| quantity == self }
      self.physical_quantity = similar.nil? ? nil : similar.physical_quantity
    end
Searches the system of known physical quantities (@@dimensions class
 array) looking for any which match self in terms of the configuration of
 base dimensions, i.e. an object which dimensionally represents the same
 thing.

 If found, the name/description of that quantity is assigned to the
 @physical_quantity attribute of self.

 This method is useful in cases where Dimensions instances are manipulated
 using operators (e.g. multiply, divide, power, reciprocal), resulting in
 a change to the configuration of base dimensions (perhaps as a new instance).
 This method tries to find a description of the new quantity.

 If none is found, self.physical_quantity is set to nil.