Figure 38. Percussion current meter invented by Julien Thoulet. Top: Number
99 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 the
number of recorded sounds per unit time was directly related to current strength
. This was tested off Monaco by Dr. Jules Richard in 1921.
| 
