halves (Fig. 18-7). You wind the coil inside one of the halves, and then bolt the two to-gether. The final core completely surrounds the loop, and the magnetic flux is confinedto the core material.The autotransformer33318-7Exploded view of pot core (windings not shown).Like the toroid, the pot core is self-shielding. There is essentially no coupling to ex-ternal components. A pot core can be used to wind a single, high-inductance coil; some-times the value can be upwards of 1 H.In a pot-core transformer, the primary and secondary must always be wound on topof, or right next to, each other; this is unavoidable because of the geometry of the shell.Therefore, the interwinding capacitance of a pot-core transformer is always rather high.Pot cores are useful at the lower frequencies. They are generally not employed athigher frequencies because it isn’t necessary to get that much inductance.The autotransformerSometimes, it’s not necessary to provide dc isolation between the primary and sec-ondary windings of a transformer. Then an autotransformer can be used. This has asingle, tapped winding. Its schematic symbol is shown in Fig. 18-8A for an air core, andFig. 18-8B for a ferromagnetic core.An autotransformer can be either a step-down or a step-up device. In Fig. 18-8, theautotransformer at A is step-down, and the one at B is step-up.An autotransformer can have an air core, or it can be wound on any of the afore-mentioned types of ferromagnetic cores. You’ll sometimes see this type of transformerin a radio-frequency receiver or transmitter. It works quite well in impedance-matchingapplications, and also in solenoidal loopsticks.Autotransformers are occasionally, but not often, used at audio frequencies and in60-Hz utility wiring. In utility circuits, autotransformers can step down by a large factor,but they aren’t used to step up by more than a few percent.