Today, elementary physics students take for granted such quantities as “big G,” the universal gravitational constant. In fact, in the late 1700s the value of this quantity was unknown, and the quest to determine it led to some of the earliest geophysical instrumentation. Just after the Revolutionary War in the United States, Henry Cavendish developed the first system to measure the universal gravitational constant, the familiar “big G.” Unfortunately, for geologists (at this time still mostly “gentlemen scientists”), this apparatus produced data which were difficult to interpret geologically, and it was far too large and cumbersome for field use. The geologic limitation was that the system measured only the horizontal derivative of a horizontal component of the gravity field, a quantity which by itself is difficult to interpret. Thus no applications of this elegant yet laboratory-bound instrument emerged.

Almost a full century later, the great Hungarian physicist Baron von Eötvös designed an instrument which would revolutionize the petroleum industry. As is often the case in revolutionizing technology, Eötvös used “new” fiber technology to significantly reduce the instrument's size and thereby increase portability. Eötvös also added a significant new feature. His master stroke was a design which suspended the weights on the torsion balance at different elevations. This modification made it possible to measure both the horizontal derivative of the horizontal field and the derivative of the vertical field (Figure 1). The vertical derivative was significantly easier to interpret geologically.