Magnetic Variometer

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.

The Cambridge Scientific Instrument Company, who supplied the Royal Observatory, Greenwich and the Magnetic and Meteorological Department in particular, with a number of instruments throughout much of the 20th century was first established by one of Charles Darwin's sons, Horace Darwin, himself an academic at Cambridge.