THE CORIOLIS FORCE

The September 1980 Popular Science brings a story "Undersea Turbines" by Dr P.B.S. Lissaman, a designer of the now famous man-powered Gossamer aircraft. The article describes turbines of monstrous dimensions that are to produce electric power from sea currents flowing through them.

The first such turbine now in the study stage is called Coriolis One and what is interesting about it is hardly its power output: With a diameter of 245 feet, it would produce 5 MW in the Florida Current of 5 knots. That makes it about as bad as windmills in power-to-size (and worse for power-to-mass).

If there were much hope for this kind of turbine, it would probably have been used to harness the power of large rivers with a slow, but voluminous flow; that would have saved the enormous effort and expense of damming them up instead.

Far more interesting is the Coriolis force, which has little connection with the Coriolis One turbine other than to modify the direction of an ocean current.

One can play ping-pong on board a train in (unaccelerated) motion, because the two players and the table all move with the same velocity relative to the ground (or some other reference point). But what happens if the train travels (radially) on some giant rotating disk? The player further from the center moves faster than his partner; and if the latter aims at some point on the table, he will miss it, because by the time the ball arrives, the table has rotated away underneath it. He looks out of the window and sees nothing strange, for everything in sight is rotating with him, so he concludes that some force must be diverting his ball from the path on which he sent it.

That force, which is just as effective as a "true" force, is the Coriolis force, named after Gaspard Gustave de Coriolis, a Frenchman who spent his life on a rotating body (the earth) and who analyzed it in a classic paper in 1835.

As the earth rotates round its axis, places in high latitudes move more slowly than points near the equator (which are further away from the axis). The Coriolis force therefore plays all sorts of interesting little tricks on us: It will deflect southbound bullets westward; it will make a stone dropped into a very deep well hit the eastern wall (the stone catches up with it); winds blowing from an atmospheric high are deflected into a clockwise rotating ("fine weather") pattern; the right banks of rivers and streams tend to be more eroded because the Coriolis force pushes the water against them; and the right rail of a one-way traffic, north-south railroad track tends to be worn out earlier than the other.

Our readers in the southern hemisphere should reverse all this (except the well); when they pull the plug to drain the bath, the eddy will usually go anticlockwise.

What the Coriolis force does is to impose a rotational pattern on the currents in the Atlantic, including the Gulf Stream and the Florida Current, which is meant to drive the Coriolis One.