Voltcraft ms 440 magnetic field analysis apparatus magnetic field tester voltcraft ms 440 p the voltcraft ms 440 magnetic field tester is intended for detecting constant and alternating magnetic fields the tester is ideal for testing the functioning of inductors carrying currents such as relays magnetic valves etc the ms 440 works with any contacts and therefore it is normally not necessary to open the casing p p if a magnet field is detected the probe will light up and an acous