pl23_ship4043.jpg
![Figure 38 ship4043.jpg Figure 38](https://s3.amazonaws.com/s3-east-testmedia-noaa.woc.noaa.gov/dam/Photos/legacy/pl23_ship4043.jpg)
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Figure 38. Percussion current meter invented by Julien Thoulet. Top: Number99 0610. Bottom: Number 99 0611. When the helix turned it caused a striker to fall on a steel plate. The sound of this was recorded by a hydrophone and thenumber of recorded sounds per unit time was directly related to current strength. This was tested off Monaco by Dr. Jules Richard in 1921.