Watson quartz fibre variometer. This is believed to be the variometer installed in the Royal Observatory's Magnetic Enclosure in Greenwich Park in about 1915 and moved as the Observatory's magnetic department was transferred to Abinger, Surrey, and Herstmonceux, Sussex.

A magnetic variometer is a device for measuring the variation in the intensity of a magnetic field. That is how each reading differs from the last or from a norm rather than given an absolute value for the field's intensity. The Watson variometer was used to measure the variation in the intensity of the vertical component of the Earth's magnetic field, while the north-force variometer made by the Cambridge Scientific Instrument Company was used over the same period to measure the intensity of the horizontal component of the Earth's magnetic field at Greenwich.

Before the introduction of the variometer at the Observatory, beginning with the Watson variometer in 1915, this type of measurement was made with a magnetometer.

Due to increasing electrical interference and vibrations from nearby railway lines, the Magnetic Department of the Royal Observatory moved to a new location at Abinger, Surrey, in 1924. Observations continued here until 1957 when the Department was moved again to Hartland, north Devon, where it continues today under the management of the British Geological Survey. Some magnetic observations were also continued at the Royal Observatory's new location at Herstmonceux, Sussex.