Two steel disks, 1 and 2, w ith annular slots a, joined together
at the centre and edges by bolts, constitute a membrane of the
pickup and, at the same tim e, one of the plates of a capacitor.
The second plate is steel disk 3, which is carefully insulated from
the first two disks. Force F bends the membrane consisting of
disks 1 and 2 downward, reducing the gap between the capacitor
plates and increasing the capacitance of the capacitor. This
change in capacitance is measured by special instruments.
$4376$SmE,M$