clamp on ferrite bead
http://paxtonlunh861.cavandoragh.org/the-most-common-complaints-about-etd39-ferrite-core-and-why-they-re-bunk-1
Besides a copper/magnetic core, ferrite cores will also be sometimes known as "double Main" cores. A double core might be useful in the production of "plate" existing supplies. When just one-core is made into a