Abstract: An apparatus for detecting variation of a condition in a mechanical device comprises a detecting transformer or plurality of such transformers provided with an input winding, a control winding and an output winding, an A. C. power source connected to said input winding, and a detecting element connected in series with said control winding. The normally closed detecting element is adapted to become open circuited in response to variation of the condition amount in said mechanical device. A required output signal is taken out and detected by an output circuit from the output winding due to the open circuiting of the detecting element.