Perikon diode

The Perikon detector has always been a bit of a famous and mythical beast:

At Fig. 11 we have the Perikon Detector developed by G. W. Pickard. This detector consists of two crystals--copper pyrites (Cu Fe S2) and zincite (zinc oxide ZnO), held in firm contact against each other in the manner shown. The copper pyrite crystal is mounted in a cup mounted on a spring-actuated rod provided with a suitable knob, by which it can be swung in any direction. Zincite crystals are mounted in a large cup containing several pockets, the mounting of both of the minerals being effected with a low fusing solder, Wood's metal or Hugonium alloy. The action of the Perikon detector is supposed to be based on the rectifying principle previously described; that is, it will pass current in one direction but not in the other, and thus the incoming radio frequency oscillating (alternating) currents in the aerial are rectified and caused to give a sound in the high resistance 'phones connected to the detector. This detector is invariably used with a battery of about two cells and the potential applied regulated by a potentiometer. When using a battery the polarity of the current must be such that the positive wire is connected to the copper pyrite crystal. - The Electrical Experimenter, January, 1917

Zincite is not common at all in Cornwall, although it is present at one smelting (slag) locality, presumably in tiny amounts. It is, in fact very rare in nature - occurring in some meteorites. It's much easier to get from synthetic sources, apparently it crystallises in chimneys - mostly from a zinc smelter in Silesia, Poland. I bought some online, and given that it comes from Poland, was only £4, and is very gemmy - it'll be from one of those chimneys.

This was a very basic, stupid test I didn't think would work, I checked the zincite was conductive, this always seems odd for a transparent mineral (it has a pretty high resistance). I paired it with chalcopyrite ('copper pyrites') from Ale & Cakes mine, Gwennap. The chalcopyrite is connected with the aluminum foil at the back making an normal 'ohmic' connection.

The result is actually quite amazing. This does not rely on a point contact at all, you need to press the crystals together quite hard for it to work. There is also no other form of conductivity, it's either not conducting or it's providing this really well formed curve. It's not leaky at all, it's very flat to the left of zero, only slightly showing some leakiness at the very end (-2.5V) which might well be an artifact.

I wonder how many materials Greenleaf Whittier Pickard tried this with before finding these results!