The flattening of the earth at the poles can be quite effectively demonstrated for teaching purposes using a device which is normally an accessory of the centrifugal force instrument. Here the earth is represented by an extremely simplified model consisting of two elastic bands of steel having a diameter of some 30 cm and set at right angles to each other. An iron rod passes through their common diameter. At one end they are fixed to the rod while at the other they are free to slide; thus, they are free to swell or flatten.
A grooved wheel at the bottom of the rod where the steel bands are attached allows the device to be connected to the shaft of the rotating mechanism.
So long as the device is not moving, the steel bands maintain their circular form but, when it begins rotating quickly, the bands assume an elliptical form on flattening out along the direction of the axis of rotation.
The model in our collection is not a part of the accessories for the centrifugal force machine described in article no. 33 both because it does not have the inventory number that appears on the other accessories and because the way of attaching it to the rotating machine is completely different.
The two rings are made of thin burnished steel strips and have a diameter of 32 cm.

Battelli - Cardani (1922), Vol. I, p. 153
Daguin (1878), T. I, p. 126
Privat Deschanel (1869), p. 67
Roiti (1887), T. I, p. 85

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