better detectors. It worked by complicated thin film surface effects, so scientists of the time didn't understand how it worked, except for a vague idea that radio wave detection depended on some mysterious property of "imperfect" electrical contacts. Researchers investigating the effect of radio waves on various types of "imperfect" contacts to develop better coherers, invented crystal detectors. Braun's experiments The "unilateral conduction" of crystals was discovered by Karl Ferdinand Braun, a German physicist, in 1874 at the University of Würzburg. He studied copper pyrite (Cu5FeS4), iron pyrite (iron sulfide, FeS2), galena (PbS) and copper antimony sulfide (Cu3SbS4). This was before radio waves had been discovered, and Braun did not apply these devices practically but was interested in the nonlinear current–voltage characteristic that these sulfides exhibited. Graphing the current as a function of voltage across a contact made by a piece of mineral touched by a wire cat whisker, he found the result was a line that was flat for current in one direction but curved upward for current in the other direction, instead of a straight line, showing that these substances did not obey Ohm's law. Due to this characteristic, some crystals had up to twice as much resistance to current in one direction as they did to current in the other. In 1877 and 1878 he reported further experiments with psilomelane, . Braun did investigations which ruled out several possible causes of asymmetric conduction, such as electrolytic action and some types of thermoelectric effects. Thirty years after these discoveries, after Bose's experiments, Braun began experimenting with his crystalline contacts as radio wave detectors. In 1906 he obtained a German patent on a galena cat whisker detector, but was too late to obtain patents in other countries. Bose's experiments The first person to use crystals for radio wave detection was Indian physicist Jagadish Chandra Bose of the University of Calcutta in his landmark 60 GHz microwave optics experiments from 1894 to 1900. Like other scientists since Hertz, Bose was investigating the similarity between radio waves and light by duplicating classic optics experiments with radio waves. For a receiver he first used a coherer consisting of a steel spring pressing against a metal surface with a current passing through it. Dissatisfied with this detector, around 1897 Bose measured the change in resistivity of dozens of metals and metal compounds exposed to microwaves. He experimented with many substances as contact detectors,