Here are some pictures of one of the older electronic components in my "collection".
It's an early 1920s valve. Valves like this were used in the very first broadcast receivers around the time the BBC was formed. This one still works, which given its age and type is probably unusual - it's a bright emitter or "r-type" so only has a few hundred hours working life. With the heater connected to a 1.5V cell it lights up like a light bulb! You can just about make out the internal construction in the photo - a thin wire filament (cathode) with a spiral grid inside a cylinder (anode).
Since there's not a lot on information on very old valves on the web here's some technical details. Manufacturer: Métal Radio (France). Model: BW 303. Markings: 1,6 - 1,8V 30 - 75V XX.
Now I must go and try and fix my AVO valve tester, it's cutting out and putting 110V across the heater terminals. Perhaps this has something to do with the 0V and 110V coarse setting for heater voltage being next to each other....