Magnetic field strength threshold indicator for use in a magnetic particle inspection device

A magnetic field strength indicator for monitoring a threshold of magnetic field strength of an electromagnet comprising:

• a coupling coil arranged to be positioned around a portion of a pole piece of said electromagnet,
• a calibration means for adjustably altering the magnitude of a voltage included in said coupling coil by magnetic flux passing through said pole piece said calibration means comprising a voltage divider connected across said coil, said voltage divider including a switch in series with the parallel combination of at least two resistors in series with a third resistor,
• a peak rectifier circuit including a time constant circuit connected across said third resistor,
• an amplifier for receiving the output of said peak rectifier circuit;
• means responsive to an output of said amplifier for providing an indication at a threshold determined by said calibration means,
• said means responsive including a relay and a light emitting diode and relay activating said diode wherever the said threshold is exceeded.