Mine is activated by the squat switch on the left gear leg.

"jsmith" wrote in message =
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Not necessarily.
Correctly, it is an oil pressure switch. When oil pressure exceeds a=20
preset value, the switch closes a contact, permitting current to flow.
On some installations, the Hobbs is wired directly to the DC buss =
which=20
is energized when the Master is turned on.
My experience is that High Performance aircraft use the oil pressure=20
switch and fixed pitch, under 200 hp aircraft use the Master switch.
=20
wrote:
Hobbs meters are driven by oil pressure.