The other day Adrian asked how rainfall was measured with my weather station. It is easier to explain with a few photos taken with a similar 'gauge' from an old defunct weather station.
To look at the rainfall measurer it is just a plastic cuboid with a hole in the top. The top surface is sloped to allow the rain water to run down to the hole:
Inside is basically a sea-saw so designed that the rain water entering the device runs to the 'bucket' which is horizontal. When the weight of rain water reaches a pre-determined amount the sea-saw flips emptying that bucket and stays in that position until the opposite bucket fills.
The measuring device consists of a reed switch - a switch operated by a magnet - which is on one side of the upright support . The magnet is on the same side as the bucket and swings with the movement of the sea-saw . When the bucket swings one way the magnet closes the switch - the opposite swing opens the switch.
The reed switch is wired to the electronics board contained elsewhere. This counts the number of times the switch opens and closes and at some pre-determined count registers 0.01 inches of rain and starts counting again.
All the complicated counting and timing is done with the electronic circuitry.
Adrian - a good project for an Arduino based rainfall gauge!!