An instrument used in magnetic surveying for detecting the presence of magnetic anomalies; it takes continuous measurements of relative vertical change in intensity of field strength. There are two detector bottles filled with water or alcohol placed at either end of a staff two meters long and held vertically during operation. Protons which form the nuclei of hydrogen atoms in the liquid gyrate or precess; the frequency of precession is identical in the two bottles if no anomaly is present. Any disturbance in the magnetic intensity caused, for example, by a buried feature, results in a different frequency in the two bottles.