What must laminations used for the core of a transformer do?

The correct choice is that laminations used for the core of a transformer must be insulated from each other. This insulation is crucial because it reduces eddy current losses in the transformer. When alternating current flows through the transformer windings, it creates a changing magnetic field that induces currents within the core material. If the laminations were not insulated, these induced currents (eddy currents) could flow freely through the core, leading to significant energy loss in the form of heat.

By using insulated laminations, each layer can reduce the circular currents that would normally contribute to energy losses, thus enhancing the overall efficiency of the transformer. The insulation between the layers ensures that the eddy currents are kept to a minimum by restricting their path, allowing the transformer to operate more effectively at the desired frequency.

In contrast, options that suggest joining the laminations or using specific materials like aluminum do not address the essential need to limit eddy currents for improved performance. The thickness of the laminations relative to the outer casing is also irrelevant to the core's operational efficiency and losses, which fundamentally relies on the insulation between the layers.