The Mach Effect Thruster (MET) is a device which utilizes fluctuations in the rest masses of accelerating objects (lead-zirconium-titanate, PZT stacks, in which internal energy changes take place) to produce a steady linear thrust. The theory has been given in detail elsewhere[1,2] and references therein, so here we discuss only an experiment. We show the stack of PZT disks being used as a thruster and then eliminate the thrust by arranging equal brass masses on either end, so that essentially the PZT stack is trying to push in both directions at once. This arrangement in theory would only allow for a small oscillation but no net thrust. We find the thrust does indeed disappear in the experiment, as predicted. The device (in thruster mode) could in principle be used for propulsion[1,2]. Experimental apparatus based on a very sensitive thrust balance is briefly described. The experimental protocol employed to search for expected Mach effects is laid out, and the results of this experimental investigation are described.