Auxanometer is used to measure

Auxanometer is the combination of two words i.e. ‘Auxain’ means to grow and ‘metron’ means to measure. Thus an auxanometer is an apparatus for measuring the increase of growth in plants.
The "rate of growth" is a derived measurement obtained by dividing the length of growth measured by the auxanometer, by the time said measurement took. It is also called an arc indicator.