17 Mar 2026
Munich is celebrating “150 Years of the Tram.” But electrification forced the magnetologists to flee the city and led to the construction of one of the most modern geophysical observatories in the world.
When the first horse-drawn tram rolled through the streets in 1876, no one suspected that the technical evolution of the tramway would seal the fate of the Magnetic Observatory in Bogenhausen. With the introduction of electric operation from 1895 onwards, the tram became an insurmountable 'source of interference' for science.
The first generation of Munich’s trams – a technologically "silent" mode of transport that emitted no magnetic interference.
The Bogenhausen Observatory on the outskirts of the city – at that time still without any surrounding development.
Munich is celebrating "150 Years of the Tram" – a milestone for urban mobility. Yet, what began as a triumph for public transport marked a drama in several acts for geomagnetic research. The technical evolution of the tram sealed the fate of the Magnetic Observatory in Bogenhausen.
It had all begun so promisingly. After the death of its founder, Johann von Lamont, in 1879, the observatory initially plunged into a deep crisis. Lamont's assistant, Christian Feldkirchner, continued the measurements, but the quality of the data deteriorated massively due to worn-out instruments. When Feldkirchner also passed away in 1886, the new director of the observatory, astronomer Hugo von Seeliger, halted the measurements altogether – his focus was on the stars, not the ground beneath his feet.
It was only the spirit of optimism surrounding the "First International Polar Year" and the founding of the prestigious Prussian observatory in Potsdam (1891) that shook Bavaria into action. Determined to end the humiliation compared to Prussia, massive investments were made: starting in 1897, new buildings were erected and state-of-the-art equipment was acquired under the leadership of Franz von Schwarz. Munich was to become world-class once again.
Historical map of Bogenhausen. The red marking indicates the expanding residential district ("Villenviertel") southeast of the Observatory and the planned tramway lines.
However, as we now know in hindsight, these decisions were tragic: while the researchers were calibrating their new instruments, the city was already planning the expansion of the electric tramway network. A new route was intended to pass in the immediate vicinity of the Royal Observatory and its attached magnetic observatory.
Franz von Schwarz came across an alarming paper by the physicist Friedrich Kohlrausch. Kohlrausch had demonstrated that the magnetic interference fields from the direct-current (DC) overhead lines of electric trams made geomagnetic measurements impossible over long distances. The observatory was only 200m away from the planned track – the researchers in Munich feared that their high-precision observations of the Earth's magnetic field would become useless.
Global seal of quality for magnetic data
Even today, great importance is attached to high-quality data: INTERMAGNET (International Real-time Magnetic Observatory Network) is a global association of observatories aiming to monitor the Earth's magnetic field with the highest precision and in real-time. For a site to be certified as an INTERMAGNET station, it must meet extremely strict requirements regarding data quality, temporal resolution, and, above all, freedom from interference.
The conduct of decisive scientific experiments today requires suitable, well-located rooms that are protected from external interference.Friedrich Kohlrausch, Physicist
Contemporary sketch of the test measurements from February 7, 1898. The terminus of the tramway line in Tegernseer Landstraße is shown at the top, while the mortuary of the East Cemetery is located at the bottom right. The plan documents the attempt to prove the long-range magnetic effect of the tram currents on the measuring station, which was approximately 200 meters away.
The scientists demanded drastic measures: on the line to Bogenhausen, only accumulator-powered trams (battery operation) without disruptive overhead lines were to be used—a technology already in use at Odeonsplatz to protect the cityscape around the royal palace. However, the city council and industry pushed for the more efficient overhead line system.
In an attempt to avert the looming end of the observatory, the danger posed by the overhead lines was to be scientifically tested. A comparable test site was found in an unusual location: the mortuary of the East Cemetery (Ostfriedhof). It was located about 200 meters from the tram terminus of that time—exactly the distance the observatory would be from the planned new route.
Since the new precision instruments in Bogenhausen were already permanently installed and too heavy for transport, only a "scaled-down solution" was used: a simple magnetometer that, like a compass, could only measure the horizontal component H. a mobile instrument for the far more sensitive vertical component Z—the crucial instrument for determining the tram's influence on magnetic field measurements—was unavailable.
This deficiency sparked a fierce argument between Seeliger and Schwarz. While Franz von Schwarz insistently warned that the experiment was worthless without the Z-component, Seeliger brushed aside the geomagnetist's concerns. Since the measurements in the mortuary showed no extreme fluctuations, Seeliger gave the green light for the construction of the tramway.
No noteworthy interference to be expected.Hugo von Seeliger, Director of the Royal Bogenhausen Observatory
The consequences followed promptly after the new tram line to Bogenhausen went into operation in 1900: precise measurements were barely possible during the day. Delicate absolute measurements could only be carried out deep at night when the trams were at a standstill. Promises made by the city to switch to battery operation in the event of excessive interference were never fulfilled.
This marked the beginning of a decades-long odyssey in search of a new measuring site far from the interference signals of an electrified major city. After a temporary stay in a makeshift hut in Maisach, the Fürstenfeldbruck Geophysical Observatory was opened in 1938. In the "magnetic silence" of the surrounding countryside, a new site for the global INTERMAGNET network was established, which continues to meet the highest standards for data quality and freedom from interference and enjoys a worldwide reputation today.
As we celebrate "150 Years of the Tram" today, we look back on a rare win-win situation. The tram made Munich mobile, and the resulting forced relocation of research to the seclusion of Fürstenfeldbruck enabled a level of precision that would never have been conceivable within the city limits. Thus, the progress of the tram paved the way for discoveries that continue to allow us to look deep into the interior of our Earth today.
Further information on the history of the observatory's multiple relocations can be found on the website of the Geophysical Observatory.