Teeth a are eq ually spaced on the periphery of counter disk 1,
which rotates at a speed proportional to the q u an tity being
measured (power, force or stress). L ig h t source 2, arranged above
disk 1, illum in ates phototube 3. When disk 1 rotates, the beam
of lig h t from source 2 is interrupted by teeth a, so that phototube
3 is subject to p ulsating lig h t w ith the frequency of pulsation
depending upon the speed of rotation of disk 1.
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