If a glitter ball falls from the Christmas tree and smashes on the floor between the gifts, then that is a result of the force of gravity. The physicist, disguised for Christmas with a red jelly bag cap knows: that ball was accelerated with 9.81 meters per square-second towards the centre of the Earth, and that is sufficient to cause it to splinter on the ground. But where does this value for the gravitational force of the Earth come from?
Exactly one hundred years ago the force of gravity was measured for the first time with such accuracy that this measured value was established as a reference value world-wide. In the Potsdam Geodetic Institute inaugurated in 1892 on the Telegraph Hill, the predecessors of the GFZ German Research Centre for Geosciences, using pendulums, measured the gravitational acceleration with such precision that this Potsdam Absolute Value attained world-wide validity.
Pendulums on Potsdam Telegraph Hill
With the publication of his report on the relative measurements of gravitation using pendulums in the time period between 1908/09 and their representation in the Potsdam gravity system, Emil Borrass introduced the Potsdam System as the first internationally used gravity reference system defining an absolute gravity level. The reference value was based on the Reversion-Pendulum measurements made by Friedrich-Jacob Kuehnen and Philipp Furtwaengler in the years 1898 to 1904 in the pendulum hall of the Geodetic Institute at that time.
This was based on experience: Robert D. v. Sterneck was first to carry out pendulum measurements in the newly established Institute. In the portly style of his time, von Sterneck remarked:"Mr. Director Helmert had the goodness to have the pendulum hall, still partly in the completion phase, provisionally equipped so that I was able to carry out the observations in this same hall. It is a special honour for me and gives me the highest satisfaction that it was I who had the privilege of being the first person to use this exemplary institute dedicated to gravity of the Earth, and that in the presence of its designer and creator, Mr Director Dr. Helmert." (v. Sterneck, 1893).
Not exact enough?
For several decades the Potsdam gravity value was the standard reference system referred to for all gravity measurements world-wide. In the 1930s measurements showed a systematic deviation from the Potsdam reference value, which was confirmed by measurements in the subsequent years. Gravity values, which are defined in the Potsdam Gravity System, lie higher by 140 µm/s². The cause for this discrepancy has actually not been fully explained to date. And the Potsdam Gravity System is also not forgotten today. Although it was officially replaced in the year 1971 by the International Gravity Standardization Net in 1971 (I.G:S.N.1971), it, nevertheless, still applies in some countries. In the case of compiling data records from different countries, these gravity data stand out due to the mentioned misalignment and must be corrected accordingly.
Dents and bumps: the Potsdam Gravity Potato
One of the most important reference fields for the Earth's gravity is the mean sea level. If one stand at the ocean's side you see a large flat surface, i,e. normal sea level. Actually, however, the sea surface shows bumps and dents, and that without wind, weather and tides.
The reason for this is the different gravity pull due to uneven mass distributions in the Earth's interior. This also pulls the water with different forces. The sea level always responds perpendicularly to the gravity pull and if this strength pulls slightly from the side, then the seacaves in at this point. South of India the sea surface shows an approximately 110 meters deep dent and north of Indonesia an 85 meter high bump. Mind you: water does not flow there, as we are dealing here with a surface of equal gravitational pull of the Earth. Researchers at the GFZ, a Helmholtz centre in Potsdam have developed a model for this: That "Potsdam Geoid" shows the irregularities in the gravity field increased by 15.000 times: a multicoloured, irregular, but dignified Christmas ball, the Potsdam Gravity Potato.
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