Abstract: Motion sensing apparatus comprise a mercury globule supported on an opaque concave surface having a transparent window at its lowest part. A photocell is exposed to light passing through the window as it is varied by movement of the mercury. The photocell output is passed to one terminal of a comparator, the other terminal of which is energized through a variable resistor by means of which the sensitivity of the comparator to the photocell input may be varied. The comparator output controls pulse generating apparatus which produces output pulses of controllable length or, alternatively, an electronically latched continuous output either of which may control mechanism of any kind.