In 2008, Williams et al. reported the discovery of the fourth fundamental passive circuit element, memristor, which exhibits electrically controllable state-dependent resistance . We show that one of the first wireless radio detector, called cat?s whisker, also the world?s first solid-state diode, had memristive properties. We have identified the state variable governing the resistance state of the device and can program it to switch between multiple stable resistance states. Our observations and results are valid for a larger class of devices called coherers, which include the cat?s whisker. These devices constitute the missing canonical physical implementations for a memristor (ref. Fig. 1).