I have located this photograph of a WWI aircraft altimeter. It looks like it has a timepiece in it. Does anyone know anything about this instrument?

Hi Craig. I have no information on this one, but it is interesting. One initial thought I have is that the winding knob may not be attached to a timepiece mechanism. When I fly one of the actions I need to take on my cockpit checklist is to set the altimeter. Altimeters work by measuring the different in barometer pressure. There is a natural increase and decrease in barometric pressure as weather fronts move through an area. One way to adjust your altimeter to account for whatever the atmospheric pressure may be that day is to know the altitude of the ground on which your aircraft is sitting and then using whatever mechanism has been designed into that particular altimeter to rotate the dial so that the indicator needle is pointing to exactly that altitude. The knob on this one may be the way you rotate the dial back and forth under the needle to set it to give the correct reading adjusted for the barometric pressure of that day in that area.

The marking Compensé would refer to the altimeter being built with a temperature compensation feature which was important for altimeters to give accurate altitude readings.

I am wondering as to whether this on is a military altimeter though. Every French military instrument I have seen so far, including altimeters, are marked Aviation Militaire on their dials and this one is not. Anything is possible though.

The outer ring of numbers on the dial would be the altitude in meters. If it is a WW I era instrument the numbers would be for hundreds of meters to make sense with the maximum reading being 5,800 meters which would have been optimistic in terms of an aircraft of that vintage actually being able to get to that height. I am puzzled by the inner ring of numbers though. Normally WW I era altimeters had such a second ring of numbers that simply continued on from the main outer ring of readings so that the needle would continue going around to inner ring readings of 6,000, 6,200, 6,400, etc. On this instrument the numbers seem to go backwards and do not follow the 5,800 number on the outer ring. Do you know what the inner ring of numbers represent?

Jim, we are fortunate that you can share your knowledge of a pilot and military timepiece collector to help us understand these instruments.

BTW, this one seems to have four mounting rings on it which suggest it was held in the aircraft with shock resistant springs. Some Longines aircraft clocks had similar mountings.

What is your source for the image?

Hi all,

re the inner ring, isnt the pressure decreasing with the altitude? Certainly 78 (780 ? mBar) is quite a low maximum but could that be a possible explanation?
rgds

The inner ring shows the air-pressure in Torr (=mmHG)(american scales are marked inchesHg); the reading of the old mercury barometers.

76 = 760 mmHg = 1013mBar

Regards,
Gerald

The photo is known to have been taken in Paris and was of interest to the Air Service of the AEF in WWI. The instrument is fitted to the leather case attached. Having done a little more research, this is quite normal. Most however do not have an outer scale that is divided into 60 units, hence the possibility that this scale may have been related to time.
So I am happy that this instrument is just an Altimeter.

Here is the photo of the case.

Thanks Gerald, Enzo, and Craig. The explanation that the inner ring is barometric pressure in Torr makes complete sense. As an American pilot I have been so used to only seeing inches of mercury where I should have been focusing on this being a French altimeter.