Ferrofluid bitter patterns on tape

Ferrofluid Bitter patterns have been found useful in the quality control of tapes and recorders. The tape fringing field induces particle density variations in the ferrofluid. The surface undulation of the dried Bitter pattern disperses visible light like a reflection diffraction grating. The relation between the surface undulation and the recorded signal has been investigated. It is found that a low distortion sine wave recording does not give clear Bitter patterns. When a dc magnetic field is applied during the development, greatly enhanced formation of the patterns is observed. Application of this technique to spectrum-analysis is described.