The Eötvös Effect

A clear explanation of this change can be given on the basis of the mechanics of Galilei and Newton. The gravitational force of the Earth is the resultant of two forces: the principal one caused by the attraction according to Newton's law, the second one the centrifugal force caused by the Earth's rotation. Since the distribution of the masses on the Earth's surface and the speed at which the Earth rotates are constant, the weight of objects on the Earth's surface is also constant. The situation is different, however, in the case of moving objects.
Instrument to demonstrate the Eötvös effect
As the Earth rotates from west to east, the centrifugal force on a moving object is greater if its motion on the Earth is towards the east than towards the west. As a result of this phenomenon the weight of a body moving eastwards will decrease, while that moving westwards will increase.

In 1915 Eötvös constructed a special instrument to demonstrate this phenomenon. The device is basically a balance with horizontal axis, where instead of pans, weights are attached to the end of the arms. The balance stands on a tripod, which rotates evenly. When the balance is rotated the weight moving towards the west will become heavier, the one moving towards the east lighter. The balance will, therefore, deflect from its state of equilibrium. If the balance is rotated at such a speed that the rotation period equals the period of its oscillation the impulses occurring during the rotations will cause the arms to make ever greater oscillations.

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