gradiometer
Concise Oxford English Dictionary © 2008 Oxford University Press:
gradiometer /ˌgreɪdɪˈɒmɪtə/
▶noun
- 1 a surveying instrument for setting out or measuring a gradient.
- 2 Physics an instrument for measuring the gradient of a gravitational, magnetic, or other field.
'gradiometer' also found in these Oxford entries:

