In order to make it possible for those interested in inventions -- especially users of technology, inventors, and patent examiners -- to discover whether inventions of a particular kind may have been disclosed and granted patent protection, it is necessary to have an organized system of classifying inventions according to the type of technology and the field of application, among other factors. Any such classification system is going to be complex and daunting to behold, and invention classification systems do not dissappoint. The the various patent classification systems are every bit as complex as the familiar system of classifying plants and animals used in biology, except that the alphanumeric coding apparently favored by technocrats lacks the clues provided by designations like chordata, vertebrata, mammalia, carnivora, felidae, panthera leo, the African lion, a roaring carnivorous mammal in the cat family with a stiff backbone.

There is an International Patent Classification system that is used, or referred to, by all members of the Patent Cooperation Treaty. U.S patents have noted the international class or classes of inventions since the early 1970s. The international class G01V 9/4 given in this patent places this invention in the field of physics, measuring and testing means, detecting masses or objects.

In the U.S. Patent Office, all patents back to 1790 have been classified under the modern U.S. Patent Classification System. Like all such systems (including the international system, above), the numbers refer to classes and subclasses starting with the general and moving to the specific. The U.S. classes given to this invention put it with electrical systems and devices (361), plural relay or solenoid load selectively (sub-191), relays and solenoids (sub-160), including means to establish distinct current levels (sub-154).

The list of classes under Field of Search indicate the other classes in which the examiner looked for disclosures of related inventions. In this case, these include classes of radiant energy technology (250), electrical communication (340), electrical transmission or interconnection systems (307), and measuring and testing of or using electricity (324).