gradiometer
noun
gra·di·om·e·ter ˌgrā-dē-ˈä-mə-tər 
: an instrument for measuring the gradient of a physical quantity (such as the earth's magnetic field)
Example Sentences
Recent Examples on the Web Matney, along with present and former UA students and Hiram students, will use a magnetic gradiometer to map subtle changes in the earth's magnetic field. 
Karen Farkas, cleveland.com, 11 Oct. 2017
Karen Farkas, cleveland.com, 11 Oct. 2017 Word History
Etymology
gradient + -o- + -meter
First Known Use
1899, in the meaning defined above