Downloaded from https://www.everyspec.com MIL-STD-1311D MAGNET POLARITY AND STRENGTH 1. Purpose. The purpose of this method is to avoid confusion in determining the strength and polarity of magnets and solenoids or magnet coils. Some tubes are sensitive to polarity, while others require only the correct shape and intensity of magnetic field. When correct polarity is required, an incorrect determination may cause immediate destruction of the tube. 2. Definitions on polarity. 2.1 The north (N) pole of a magnet is that pole which points toward the geographic north of the earth, when the magnet is freely suspended. No distinction is made between "north" and "north-seeking".
The implication that the earth's pole in the north of Canada is a magnetic south (S) pole is an accepted paradox. 2.2 The direction of a magnetic field at any point, whether due to a permanent magnet or to a solenoid, is the direction of the force on the north pole of a small magnet at that point. Hence, it is the direction in which the N end of a compass needle points when placed in the field. 2.3 The direction of the external field of a permanent magnet is away from the north pole, toward the south pole.
However, the ends of a solenoid should not be marked N or S, because both the internal and external fields are accessible, and run in opposite directions, so that any such marking is ambiguous. Coil markings, if used, should relate the direction of the internal field directly to the polarity of the current supply, without reference to N or S poles. For example, METHOD 1367A "Internal field direction when black lead is positive." 3. Method for polarity. 3.1 Permanent magnet: Bring a compass or suspended test magnet near to one end of the permanent magnet. This end is north (N) if the compass points away from it. 3.2 Solenoid or coil:
With a current of known polarity flowing in the solenoid or coil, bring a compass into the region of interest (normally inside the coil*); the field direction is the direction in which the compass points, assuming that the current is sufficient for the solenoid field to mask the earth's field. * Even when the region of interest is inside, it may be more convenient to check the polarity externally (provided there is no iron return path), to avoid removal of a tube or other component within the coil.
In this case, the polarity should be checked close to the coil and in any plane, perpendicular to the axis, which passes through the working region (near the ends the field has strong radial components). The internal field direction is then the opposite of that observed externally. 3.3 If the winding direction of a coil is known or can be seen, the magnetic field direction can be determined by the right-hand rule: With the right thumb along a wire, pointing in the direction of current flow, the fingers curl around the wire in the direction of the magnetic field.
Alternatively, to an observer looking along the coil axis, a clockwise flow of current produces a magnetic field directed along the coil axis and away from the observer. 4. Precautions. A compass or small test magnet can be remagnetized in the reverse direction if it is suddenly exposed to a large field from a permanent magnet or solenoid. Therefore, the compass or test magnet should be checked before and after a measurement by taking it a short distance away from local magnetic influences, and making sure that the N or marked end points toward geographic north. 5. Convention for magnetrons.
Non-packaged magnetrons (for example, magnetrons operated in a permanent magnet which is separate from the tube) should be mounted with the cathode terminal of the tube adjacent to the north pole of the magnet. METHOD 1367A 1367-1 25 March 1970