Auxanometer

[1] In case of an arc-auxanometer (see picture), there is a thin cord fixed to the plant apex on one end and a dead-weight on the other with a pointer indicating against an arc scale.

In some forms it passes over a pulley which has a pointer attached to it.

When the plant's height increases, the pulley rotates and the pointer moves on a circular scale to directly give the magnitude of growth.

These simple types of auxanometer have been replaced by rotation sensors at the fulcrum point linked to dataloggers with a balancing beam attached to the growing tip/plant apex.

Sensitive auxanometers allow measurement of growth as small as a micrometer,[3] which allows measurement of growth in response to short-term changes in atmospheric composition.