Wondering how a whistle rectifier disc works?

[mikemc52]

I have had some recent whistle operation problems (post war equipment) and through some very helpful tips to a post I wrote I decided to purchase some new rectifier discs for my post war ZW transformer.  Please bear with me I'm a mechanical engineer and not a sparky, so electricity is still magic to me.  I know what a rectifier is and what a diode is but for the life of me I can't imagine how the rectifier disc that doesn't look like much more than a washer actually works.  Can anyone explain the physics of these things?

Thanks, Mike 

[webenda]

The selenium and aluminum form a PN junction. Electricity passes in one direction and is blocked in the other direction. Copper oxide and lead can also be used to make a metal rectifier. Selenium assemblies can withstand about 20V in the reverse direction and can be stacked to achieve higher voltages. Note-- The selenium is a grey form of the element but I made it red for better visibility in a small image. The aluminum contact is actually aluminum oxide.

[mikemc52]

Thanks for the great explanation!

Mike

[webenda]

Another way of forming the PN junction is with-in layers on a steel or aluminum washer. A thin layer of bismuth or nickel is deposited on the washer Then a layer of selenium followed by an upper electrode consisting of a tin-cadmium alloy. The boundary between the selenium and upper electrode forms the junction.