To connect the secondary coils for series operation on a transformer with four secondary terminals, which terminals are joined with an external jumper?

In a transformer with four secondary terminals designated as X1, X2, X3, and X4, connecting the secondary coils for series operation involves linking certain terminals to create a proper path for the current to flow through both coils effectively.

When connecting in series, one of the primary considerations is maintaining the same phase for the voltage from each coil. Joining the terminals X2 and X3 achieves this because it connects the end of one coil to the start of the next coil in the correct phase orientation. This configuration results in the addition of the voltages produced by each coil, allowing the transformer to operate at a higher voltage output.

By linking X2 and X3, the circuit effectively combines the outputs of those secondary coils, ensuring that the resulting series voltage is the sum of the individual coil voltages, maximizing the output capacity of the transformer in series operation.