From: gherold@teachspin.com   
      
   On Wednesday, May 9, 2018 at 1:37:19 PM UTC-4, Robert Roland wrote:   
   > I have been thinking about the way a variac works. One thing that   
   > puzzles me, is the wiper design:   
   >   
   > If the wiper is too narrow, the output voltage will cut out when the   
   > wiper is between two turns of the coil.   
   >   
   > If the wiper is too wide, it will bridge two contact points, causing   
   > one turn of the transformer to be directly shorted. Although it is not   
   > a lot of voltage, it is also not a long piece of copper, so the short   
   > circuit current, I expect, would be considerable.   
   >   
   > Obviously, getting the wiper the perfect with, and also the wire   
   > spacing equally perfect is not a practical approach.   
   >   
   > How to they do it in practice? Is there some kind of snap action that   
   > makes the wiper click from one turn to the next?   
   > --   
   > RoRo   
      
   The small one I have here, (1.75 A) has a wiper that spans at least   
   three turns at a time, but there's ~1000 (?) turns on the thing.   
   (Staco type 171) I couldn't find a good image.   
      
   George H.   
      
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