A differential magnetic circuit (magnetic bridge) is described. The circuit separates the magnetic field sensor and the sample under study. A Hall probe serves as the sensor. The signal from the sensor can be enhanced by concentrating the magnetic flux. The magnetic bridge works even with dc magnetic fields. The device is used for displaying hysteresis loops of ferromagnets and for observing the superconducting-to-normal phase transition in a high-temperature superconductor and the Curie point of a ferromagnet. The magnetic bridge seems to be useful for undergraduate laboratories in electricity and magnetism. (Contains 6 figures.)