In a vibrating sample magnetometer the signal contains a spurious component due to the ac magnetization induced in the sample as it vibrates in an imperfectly uniform dc magnetic field. In high‐conductivity and superconducting samples, the spurious signal may be larger than that due to the dc magnetization. A quantitative analysis is given, and measurements made in an uncompensated solenoid are compared with those made in a fourth‐order solenoid.