Despite its extreme linearity and accuracy, this remarkable circuit design is all but forgotten today. It deserves greater recognition.Certain electronic components share a name ending in –tron, such as the ignitron, magnetron, thyratron and klystron. Each of these could be described as peculiar parts, which is why the klystron has already been covered in this series, as was the Dekatron, Trochotron… Unlike the other –trons, the subject of this article is not a physical component but a very peculiar and special kind of circuitry.