A well-known case from the world of welding illustrates the Doctrine of Equivalents [source: Blenko]. In this case, an existing patent included claims describing welding rods with a flux, or coating, containing alkaline earth metals, a series of elements containing beryllium, magnesium, calcium, strontium, barium and radium. Another manufacturer tried to introduce a new rod with a flux containing primarily manganese, which isn't an alkaline earth metal. Nevertheless, the courts decided that the accused welding rods infringed on the existing patent because manganese and magnesium were similar enough in many of their reactions that they could be considered the same, or equivalent.
What It Means to Infringe
It may seem strange, but the founding fathers set the stage for patent infringement when they included language about patents in the U.S. Constitution. Here's what Article I, Section 8 says: "Congress shall have power … to promote the progress of science and useful arts, by securing for limited times to authors and inventors the exclusive right to their respective writings and discoveries." You can read more in How Patents Work, but here's the bottom line: A patent is, in essence, a limited monopoly. It gives an inventor the sole right to make, use and sell his or her innovation for a set period of time.
The very existence of a patent, however, sets the stage for battles. What if two people come up with the same idea simultaneously? That exclusivity the Constitution promises patent holders isn't an ironclad guarantee of protection.
In fact, the onus is on inventors to make sure they file a solid, defensible application with the government agency that oversees intellectual property. In the U.S., that body is the U.S. Patent and Trademark Office (USPTO). When inventors apply for a patent, they must describe the claims of the patent very carefully and precisely. This language determines what can and can't be protected. Infringement occurs when the infringer tries to make, use, offer for sale, sell or import an invention that falls within the claims of an existing patent.
Sometimes, the infringer knowingly tries to copy an existing product or process, perhaps with a minor modification to make it seem different enough to be considered a new invention. This comes down to a question of equivalency, which the Supreme Court defines simply: "If two devices do the same work in substantially the same way, and accomplish substantially the same result, they are the same, even though they differ in name, form, or shape" [source: Blenko]. You can read more about the doctrine of equivalents in the sidebar.
At other times, one inventor, with no knowledge of similar efforts, comes up with the same idea as another inventor. This is known as independent invention, and though you might think patent law would treat such cases less harshly, that's not always the case. In fact, independent invention can't be used as a defense against patent infringement. One of the most famous examples of this is the telephone. Both Alexander Graham Bell and Elisha Gray filed paperwork to register their competing telephone technologies on the exact same day. Neither was the first to come up with the idea, however. Italian immigrant Antonio Meucci had already conceived of the telephone years earlier, but didn't have the money to file a patent with the USPTO. Ultimately, Bell got credit for the invention because he was faster out of the gates. Although disappointed by the decision, Gray went on to become a professor of dynamic electricity at Oberlin College [source: PBS].
So what happens if an inventor, or patentee, believes his or her invention has been infringed? Often, it's off to court, which is where we're headed next.