a diagnostic filter device for monitoring electrically conductive and semiconductive particles entrained by a fluid , composed of : a filter element disposed to receive the fluid and to permit passage of the fluid while retaining particles entrained in the fluid ; a component disposed for producing an alternating electromagnetic field which extends across at least one region of the filter element ; and an eddy current detecting instrument connected to the electromagnetic field producing component for detecting the influence on the field of particles in the filter element region .