We present an original method for field stability measurements in a bath of liquid helium. This method is used to validate the power supply of a superconductive magnet operating in driven mode. The experiment consists in the measurement of the NMR signal of a sample of liquid 3He, placed inside the field of a test magnet driven by a power supply. The homogeneity of the magnet is a strongly limiting factor for measurements but through the use of an inductively coupled microcoil and careful signal processing, a precision of 5.5 ppm was achieved.