Final answer:
To distinguish the primary winding from the secondary in a step-up transformer, you can look at the number of wire turns—fewer on the primary compared to the secondary for increasing voltage, following Faraday's induction law.
Step-by-step explanation:
The external switch on a dual transformer changes the primary coil by selecting different tapped points or winding configurations, enabling variable voltage levels. To distinguish the primary winding from the secondary winding in a step-up transformer, you can look at the number of turns in each coil. The primary coil typically has fewer turns of wire than the secondary coil if the transformer is designed to increase voltage. In contrast, if the transformer is a step-down transformer, it will have more turns on the primary than on the secondary. This relates to Faraday's law of induction, where a change in current through the primary induces a current in the secondary coil due to a time-varying magnetic flux created by the alternating current (AC) in the primary.
The fact that the primary and secondary coils can have an equal, greater, or fewer number of turns depending on the desired voltage transformation clearly illustrates the versatility of transformer design. Additionally, the core material, which is often laminated to reduce eddy currents, plays a critical role in increasing the magnetic field strength, enhancing the efficiency of the transformer.