Fonctionnement - Solar pendulum

This pendulum works only by direct sunlight.

(1) Put a roll of black paper in the test tube. Fill the test tube with water, then insert the small tube inside. Push very well the stopper and wipe off any spilled water.

(2) Screw the candy bottle on its cap. The test tube should not touch the sides of the bottle. Fix the pendulum on its stand by placing two paper clip "bars" both sides of the cross junction.

(3) Place the mobile against the Sun and set its angle so that the reflector is perfectly perpendicular to the Sun (the shadow of the winged nuts should be at the base of the metal rods).

If necessary, add a little water through the hole on top of the ping-pong ball so the pendulum is nearly tipping over (you can use a piece of small tube and suck some water in, keep the water in by closing the top hole with your finger until you release it in the ping-pong ball).

If the mobile tips forward but does not return it is often caused by a too heavy ping-pong ball. Try to empty a little bit the ping-pong ball.

How does it work ?

Thanks to the greenhouse effect and absorption of sunlight rays by the black colour, the water warms up in the test tube.

The water expands and climbs along the tube towards the ping pong ball.

The weight balance is changed and mobile tips over.

The test tube in then no more in the focal point. It cools down and a contraction is created.

Water flows back to the test tube and the weight balance is changed again. The mobile tips back.

The cycle can now start again.

Here is a small video the solar pendulum in action.

The rythm of its moves is variable, but never very fast.