Hobson Telegage                  

The Telegage uses the sloshing of petrol in the tank to maintain an air column within a vertical pipe in the tank, which would then be at a pressure equal to the hydrostatic pressure at the open (bottom) end of the pipe.  This pressure was then indicated on the dashboard using a manometer, as seen in the images below.  There are two types found on surviving FWD and it is uncertain whether both are original.  The fluid for the gauge is tetra-bromo-ethane C2H2Br4, (AKA acetylene tetrabromide, or Muthmann's Liquid), which is nearly four times denser than gasoline, so the manometer does not have to be as tall as the depth of your tank.  The liquid should be dyed red, the original dye being Sudan Red, an aniline dye used to colour waxes, solvents and gasoline, also used in laboratory work. 

Resist the following temptations: 

(1) drinking the tetra-bromo-ethane, or sniffing its vapour.  See Material Safety Data Sheet here, courtesy of Mallinckrodt Baker, Inc.. 

(2) eating the Sudan Red - it has been banned as a food colouring as it is a carcinogen.

The manufacturer's description and servicing instructions are to be found at this link:   Hobson Telegage Instructions  and better quality images are found here:  Telegage images

Click on thumbnail to view higher resolution image.  Right-click to save to file.

Chassis 7308.

An original gauge, but with bezel missing.

Chassis 7315.

Probably, this is the correct pattern fuel gauge. This type is also seen on the Alvis 12/60 of 1931.

Chassis 7313.

Original gauge, bezel missing.

Chassis 7129.

Another instrument that is believed to be original, as it is similar to those seen on the 12/50 of the late 1920's. This type has a wide die-cast metal bezel, usually a dull grey in colour.

Chassis 7256.

Probably, this is a later replacement, as it has a differently calibrated face from the standard, and the bezel is of the wider form but not quite the same shape as that on chassis 7129. This is nonetheless a suitable instrument.

1927 Hobson advert detail.

This seems to confirm that the wider bezel was a slightly earlier design.

1927 Hobson advert complete.