Hot-cathode ionization sensors were very similar to vacuum triodes. Their operating principle is based upon measuring the ion current in the collector, which decreases as vacuum increases. The grid is biased positive with respect to filament, attracting and accelerating electrons emitted by the filament. The anode is biased negative and acts as collector for ions derived from impacts of electrons on gas molecules. In the Bayard-Alpert sensor the geometry of electrodes is modified for better efficiency. The filament is mounted outside the grid helix and the anode is a small filament in the middle.
The structure is: outer mesh cylinder, elliptical grid and single strand wire. This looks to be the standard triode design but the source indicates that the 29D15 is the modern Bayard-Alpert design with the filament on the outside.
The concentric electrodes and open glass cylinder.
The central wire is tensioned by a spring - filament fashion. The central grid can be seen near the base and the shadow can be seen within the mesh. The grid winding is the same diameter along the whole length.
The thin glass tube envelope is 29 mm in diameter and, excluding the B9A base pins, is 49 mm tall.