A triode valve consists of a heated cathode that emits electrons which are attracted to a positive anode. The current is controlled by applying a small potential to a wire grid situated between the cathode and the anode. The whole is enclosed in an evacuated glass vessel. The valve is the basis of an amplifier. The triode was invented by de Forest in the U.S., with a patent filed in 1905. The valve here can be identified as one of the R-type valves produced in the early 1920s. It has a straight directly-heated cathode and a helical grid. The four-pin base became standard around that time.