Compensated Holosteric Barometer with Zeroing Device 0-2000m, OTTO BOHNE NACHF. BERLIN S., circa 1914

Compensated Holosteric Barometer, produced by "OTTO BOHNE NACHF. BERLIN S." around 1914, to be installed on bomber aircraft (Bombenflugzeug) supplied to the "Liegertruppen des deutschen Kaiserreiches" (in english: German Empire Air Troops), a name which remained until October 1916, after which it changed to Luftstreitkräfte.

This fundamental instrument, which detected altitudes between 0 and 2000 meters, is equipped with a very rare zero pointer (or zeroer) on which is written "feststellung drucken", placed in the center of the dial which communicates and acts on one of the two "hands" of the instrument.

To understand the importance of this small device, we must go back to basic flight knowledge: it is not the absolute flight altitude above sea level that must be considered but the altitude above the airfield during the approach to landing, therefore a relative altitude. In fact, relative altitudes are determined by the height from the ground compared to the flight altitude and were very important, especially for bombers, to be able to calculate the exact time in which the bombs had to be dropped in order to hit a certain target. Therefore, through this instrument, the pilot set his altimeter to 0, an operation called zeroing, in order to exclude the altitude above sea level automatically detected by the barometer. In fact, these instruments, even if the aircraft was on the ground, still detected the altitude of the take-off/landing runway or airfield compared to sea level. This accessory instrument was therefore used to manually act on the 0, thus providing the pilot or bombardier with the real altitude above the ground. A different procedure could be applied if the landing was to take place in a different place and at a different altitude than the take-off. The pilot set the altitude of the take-off place on the altitude scale using his altimeter, if he then decided to land at another known altitude, he did not have to land at 0, but for example, at 450 meters: even in this case, in the adjustment, the atmospheric pressure had to be taken into account, etc.

A concomitant phenomenon of this type was that during flights, then very short and included in a time of 1-3 hours, the displayed altitude corresponded to the absolute altitude, this represented a small advantage when flying over obstacles with known heights.

This instrument was therefore not only useful for knowing the altitude above sea level or from the ground, but was essential in establishing the release time of the bombs so that they could hit targets with extreme precision. Furthermore, the maximum altitude recorded of 2000 meters, implied that the maximum release altitude should not be higher: doing so, increased the risk of being hit by enemy antiaircraft fire, but drastically reduced the margin of error in the timing of the bomb release.

An original period photo shows this version of Höhenmesser.

Otto Bohne, after working in Paris, perhaps for Naudet (PHBN), founded his own company in Berlin in 1863. His barometers were copies of PHBN barometers, perhaps produced under license since they are "holosteric" (not liquid) which at the time, was a unique feature adopted by PHBN on their barographs. Otto Bohne, invented methods of making temperature compensated aneroid barometers. His first barometers were made for use by scientists, meteorologists, etc. etc. and only later did Bohne produce barometers for the civilian market. The company produced, from 1890, barometers used in balloon flights and altimeters used in aviation during the First World War. The company exhibited its instruments in Berlin in 1879, in Chicago in 1893 and again in Berlin in 1899.

After Otto Bohne's death in 1906, his successors were Louis Kurth and Otto Kohlman who renamed the company "Otto Bohne Nachfolger" (successors). They exhibited their production in Brussels in 1910. The last known mention of this company dates back to around 1927.