A geophysical instrument that measures the intensity and sometimes direction of the Earth's magnetic field. It is used in electromagnetic surveying to identify changes in the field within soil or sediment that might be caused by subsurface features, hearths, kilns, or metal artifacts. When a current is passed through a coil in a bottle of water or alcohol the protons of the hydrogen atoms align themselves to its magnetic field. When the current is cut off, the protons realign themselves according to the earth's field, its strength being indicated by the frequency of their gyration on realignment. This sets up a weak current which is transmitted back from the bottle to the instrument and there registered on dials. The resulting figures are plotted to reveal anomalies in field strength - usually due to buried iron, kilns, hearths, or to pits or ditches. These features can thus be rapidly located without disturbance of the ground, and excavation can be directed to the most promising areas. Magnetrometry is the use of a magnetometer for mapping subsurface anomalies. There are a number of designs, but two are particularly widely used. The proton magnetometer makes an absolute measurement of field strength, but is intermittent in operation: each reading is initiated by the push of a button, and takes some seconds to appear on the display of the instrument. Fluxgate magnetometers work on a different principle, and give a continuous reading, which makes surveying less time-consuming. Most fluxgate machines do not however measure field strength directly, but rather are gradiometers, measuring the vertical gradient of the earth's' magnetic field, i.e. how fast the field strength changes with vertical distance from the earth's magnetic field Gradient measurements can also be used in archaeological surveys and have an advantage over absolute measurements. The earth's field strength varies continuously during the day at any one location. Absolute measurements taken at different times have to be calibrated for this effect if they are to be comparable. Gradient measurements are not affected by this diurnal drift in field strength, and so do not need to be calibrated. Proton gradiometers are also available. The fluxgate, differential fluxgate, and proton gradiometer take continuous measurements of relative vertical change in the intensity of field strength.