The long‐time stability of an inhomogeneous magnetic field is measured with a precision of 3×10−5. The system is instrumented for short‐time dependence on component stability while referencing on a precision homogeneous field with NMR. An InAs Hall effect sensor is positioned in the gap of a permanent magnet assembly. The Hall voltage is balanced by a low voltage reference source. Without changing the reference voltage the Hall probe is placed in the field of a 6‐in. electromagnet whose field is varied to again obtain null, then is measured with an NMR gaussmeter. The time requirement on component stability is about one minute. The Hall probe is temperature controlled to ±0.01°F.