# the Science Centre and Exploratorium in Trondheim, Norway.

### The ball clock counts the time using ping pong balls.

Every minute a fan starts up and blows a new ball from the reservoir at the bottom, through the long tube and up to the uppermost tray. This tray has room for two balls. When it is full, it tilts. One ball rolls onto the second tray, the other falls to the reservoir. Thus a new ball enters the second tray every two minutes. The second tray has room for 5 balls. When it is full, it tilts. One ball rolls onto the second tray, the others fall to the reservoir. The third tray has room for 6 balls. Thus the three uppermost yellow trays count the minutes in an hour. Each ball in the uppermost tray counts for one minute, each ball in the second tray counts for 2 minutes, and each ball in the third tray counts for 10 minutes. If there is 1 ball in the uppermost tray, 2 in the second and 3 in the third, the time is 1+2x2+3x10=35 minutes past the hour.

Every hour a new ball enters the fourth tray, which has room for 4 balls (each ball counts for one hour). Every fourth hour a ball enters the fifth tray which has room for 6 balls (each ball counts for one hour). Thus the two lower black trays count the 24 hours in a day. If there are 3 balls in the fourth tray and 2 in the fifth, the hour is 3+2x4=11.

The counting mechanism is made entirely from Lego Technic. It is sponsored by Lego Norge. The time base is electronic, and a microcontroller runs the fan (at the lower left) for 5 seconds every minute.

At the lower right you can see a small ball counter that counts to 3x3=9. It does not require large amounts of Lego, and should be within reach for many children who have got some Lego at home. My youngest son, Bjørn, aged 7 at the time, experimented with such clocks while I made the large.

There is also a large rolling ball sculpture at "Vitensenteret".
Here are more pictures of it.