That BradtheRad's construction is essentially a home-made current transformer. There is no magic in making them self, indeed. Still the physics apply: Assuming, that the transformer is made "right enough" to have good magnetic coupling, the current is transformed by ratio of the windings. To use such transformer as a current transformer, one has to load it heavily, by having a relatively low resistance load on it. "Relative" means, that depending on current to be measured and the transformer windings ratio (secondary/primary), the resistance should be heavy load/virtual short-circuit for the transformer. Then the voltage over that resistor presents the current, being U=(n1/n2)*I*R where n1 is the primary number of windings, and n2 the secondary number of windings. In a pass-through-the-wire -type current transformer n1=1, and n2 is whatever is wound around the core for secondary (the core is often a toroid for good magnetic coupling).