http://www.tvrepairinfo.com/ Here is a link to Jestine Yongs article on the Blue Ring tester. http://www.electronicrepairguide.com/blue-ring-tester.html
In this video I demonstrate how useful a ring tester can be when your looking for shorted turns of wire within a Transformer or Inductor. Even though I've had one of these testers for many years It wasn't until recently that my friend Steve helped me fully understand what advantage it would have in testing a coil for shorts compared to a low resistance Ohmmeter. The main advantage is being able to find shorts an Ohmmeter isn't likely to pick up, and being able to know your coil is shorted even though you may not know what the resistance of your coil is suppose to be. Here is a link to Jestine Yongs article on the Blue Ring tester. http://www.electronicrepairguide.com/blue-ring-tester.html