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Magnetic Measurement

The lock-in amplifier has proved a useful tool when making measurements on magnetic samples. It forms the heart of the Vibrating Sample Magnetometer, in which a sample is vibrated vertically between two poles of an electromagnet or superconducting solenoid. Pick-up coils that are adjacent to the poles detect the disturbance in the field that occurs when the sample has any magnetic moment, with the size of the signal being proportional to such moment. A lock-in amplifier measures this signal (which is at the vibration frequency), and by sweeping the magnetic field value it is possible to plot the B-H curve of the sample. The noise rejecting capability of the lock-in amplifier ensures that interfering signals (especially at the power line frequency and its harmonics) are rejected and do not affect the measurement.

Lock-in amplifiers
are also used to detect signals from Hall effect devices when these are driven by an AC excitation signal. Using AC excitation eliminates the drift and offset problems that can occur when measuring using DC techniques, as well as allowing measurement of smaller signals.

Signal Recovery supplies one of the widest commercially available ranges of lock-in amplifiers, as well as associated products such as a range of preamplifiers and signal transformers that are suitable for these types of applications. Typical instruments for this application used are: