Gravity Measurement Based on a Levitating Magnet

taylorvich writes:

https://physics.aps.org/articles/v17/48

Gravity measurements can help with searches for oil and gas or with predictions of impending volcanic activity. Unfortunately, today's gravimeters are bulky, lack stability, or require extreme cooling. Now researchers have demonstrated a design for a small, highly sensitive gravimeter that operates stably at room temperature [1]. The device uses a small, levitated magnet whose equilibrium height is a sensitive probe of the local gravitational field. The researchers expect the design to be useful in field studies, such as the mapping of the distribution of underground materials.

Several obstacles have impeded the development of compact gravimeters, says Pu Huang of Nanjing University in China. Room-temperature devices generally use small mechanical oscillators, which offer excellent accuracy. However, they are made from materials that exhibit aging effects, so these gravimeters can lose accuracy over time. Much higher stability can be achieved with superconducting devices, but these require cryogenic conditions and so consume lots of power and are hard to use outdoors.

Superconducting systems have exceptional sensitivity partly because they use levitation-a small oscillating system is made to float in space and so remains free from many disturbances, such as vibrations, Huang says. He, along with Jiangfeng Du of Zhejiang University in China and their colleagues, wanted a design that would also employ levitation but without the cooling requirements of superconductivity or the need for materials that age.

Their new device is based on a magnetic-levitation concept developed more than two decades ago [2]. It involves two magnets-a large magnet, fixed in position, and a smaller, 200-mg test magnet located a few centimeters below, with field-repelling (diamagnetic) slabs of graphite positioned above and below the test magnet. The upward force supplied by the fixed magnet balances the weight of the test magnet, so it can levitate. The slight repulsion between the test magnet and the two graphite surfaces allows the magnet to oscillate stably in the vertical direction. The team adjusted the spacing between the surfaces to reduce this oscillation frequency to about 1 Hz. (The lower the frequency, the more sensitive the measurements can be.)

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