The Radio Micro Bi-Volt is a general purpose triode from 1926. The identification is a well formed but very faint etched lettering invisible under normal conditions. What special illumination reveals is that the glass gives the maker's name as Dario. Radio Micro (Sociiti La Radiotechnique) became Dario in 1927 and so this exhibit must date to 1927 or later.
The 1.8 Volt filament allows for a rheostat in series with the two Volt accumulator LT supply.
The construction has a horizontal cylinder anode with a helical wire grid and a central single strand filament.
A closer view of the electrodes. It can be seen that the grid loops are fixed to the support bar. The latter is held in two places and is firmly attached to the pinch. The single strand filament is broken.
A pleasant surprise to actually see this lettering.
The wide glass tube envelope is 38 mm in diameter, and excluding the base pins is 77 mm tall.
Reference: 1043. Type Bi-Volt was first introduced in 1926.