In 1967 Dale W. Evertson of the Applied Physics Laboratory, University of Texas, visited Sacks to demonstrate a curious yet remarkable instrument, called a solion. It was capable of detecting the flow of extremely small ion currents in solution between two electrodes and had been developed as an acoustical detector for very low-frequency waves, the outgrowth of anti-submarine research that had found it of no use. Impressed by its sensitivity, Evertson searched for an application and constructed a seismometer with it, hence the visit. Sacks found it unimpressive as a seismometer but saw in the solion another application and initiated a field of study that was to occupy both men for the remainder of their careers.

Ever since earthquakes had been associated with faulting there had been a general belief that the key to understanding, even predicting them lay in determining the strain that built up in crustal structure. This naturally led to the invention of a variety of devices for measuring strain. These were generally based on the accurate measurement of the distance between two piers in a cave or tunnel having little temperature variation. From these efforts beginning in 1900 little of substance had come for reasons soon to be explained. Sacks saw in Evertson's device a new, and as it proved, successful approach to this old problem.