http://durealeyes.com/planimeter.html

coke can planimeter


A planimeter measures area; this is one made from a soft drink can.

[planimeter]
The pointer projecting horizontally from the right-hand end of the
can is moved round the shape to be measured, the total rotation of
the can is proportional to the area. All the different coloured
shapes (triangle, circle, two oblongs) have been drawn the same area.
So moving the pointer round any one of them will cause the can to
rotate by exactly the same amount.

[planimeter]
Only one end of the can touches the paper, I found it works better if
you put PTFE tape round this bit of the can. The blue-tack weight
also seems to improve the accuracy, I stuck a bent paperclip in it to
help read the rotation of the can. A bit of bent wire runs through
holes in either end of the can & is bent down in a right-angle at the
left-hand end where it is sellotaped to the bent card to form a
hinge-bearing.

[planimeter]
The other end of the card has a drawing pin stuck through it which is
sellotaped to the table. You have to use both hands to work it: the
left hand moves the cardboard linkage while the right hand guides the
pointer round the shape. It's important that the right hand exerts no
vertical force on the can. It's difficult to use but gives quite
consistant results. It could be made more sophisticated by sticking a
spiral paper scale to the can, so that the amount of rotation could
be read, rather than just showing that different shapes had the same
area.

Link to movie



plastic spoon planimeter


This is a much simpler way of measuring area

[spoonplani]
If the right-hand end is carefully traced round an outline using a
pin & sellotape as a pointer the vertical displacement of the
left-hand end is proportional to the area. This kind of works if you
just push a teaspoon round carefully with your finger.

Link to movie


hatchet planimeter
Douglas Brown's explanation
mechanical computing
2000 year-old computer
DigiComp
make your own drawing instruments



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