What is an Impeder? How does it work?
The primary function of an impeder is to increase the impedance of the parasitic current path around the inside circumference of the tube, thus diverting more of the available energy into the weld "vee". Impeders also concentrate the magnetic flux created by current in the work coil, so that a greater amount of energy is induced into the tube. Below are a few links to the different styles of impeders that EHE manufacturers.
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