A magnetic field strength threshold alarm that includes sensing means responsive to a magnetic field and actuating in response to field strength above a predetermined threshold, the sensing means being configured to be operational and able to actuate without consumption of energy； and alarm means for outputting an alarm responsive to the sensing means actuation, the alarm means being configured not to consume energy prior to actuation of the sensing means and only consuming energy subsequent to actuation of the sensing means. So, energy is not consumed by the alarm means prior to actuation. The alarm may be considered to include energy storage means for providing electrical energy, wherein the sensing means, the alarm means and energy storage means being operatively connected such that the electrical energy from the energy storage means is provided only when the sensing means is actuated.
