I obtained surplus from somewhere (likely Naval Postgrad. School) an old circuit board (phenolic fiber composite --thick) with two 6H6 sized octal VTs and Rs and Cs. The tubes are labeled OTIS 2040. I googled and found they are Ne filled diodes, which evidently conduct from the capacitance of ones finger near the top of the envelope. One of the elements is a wire, insulated by a glass tube from the base whose end is near the envelope top. The other element is an inverted metal cup about the middle of the tube. One of the pages wrote the two tubes are selected for equal sensitivity and the EMF is labeled. ** One of the tubes is labeled (pasted paper) TD 73 and the writer wrote the TD 73 tubes are operated at 73V. One writer mistakenly? called them thyratrons.