Historical Background
This valve appears to be one of the approximately fourteen valves which Prof. J.A. Fleming experimented with, and examined the Edison Effect, in 1889-90. This was while he held the post of Professor of Electrical Engineering at University College, London (UCL), and 10 years before he was appointed chief scientific advisor to the Marconi Company. In 1904, Fleming took three of these valves out of a cabinet in which they had been stored, to experiment with the use of the valve for radio detection. Denman illustrates nine of the remaining valves (1924): the three used for the 1904 experiments are shown in the frontispiece to Fleming's own work "The Thermionic Valve and its Development in Radiotelegraphy and Telephony, 1919".