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

wrote:
Hobbs meters are driven by oil pressure.