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Quote Originally Posted by autoslot View Post
Actually a Variac does not have a rheostat. It is what is called a variable autotransformer. Most transformers use two sets of coils and have electrical isolation between the low voltage and high voltage side. An autotransformer has only one coil of wire. The Variac has one input wire on one end of the coil and the other input wire about 86% down the coil (120/140). The first output wire is tied to the first input wire (neutral) and the other output wire is connected to a wiper that can run the length of the coil, thus providing 0V to 140V output.
I agree, a Variac is definitely not a rheostat, which is why you can achieve a step-up in voltage. With a rheostat, you can only go down.