Final answer:
If the tap is raised and the primary voltage drops, the number of turns in the primary coil must be increased to maintain the output voltage, as the voltage ratio is directly proportional to the turns ratio in a transformer.
Step-by-step explanation:
When the tap is raised on a transformer and the primary voltage drops, the number of turns of the primary coil is increased to maintain the output voltage. This adheres to the principle that the ratio of the secondary to primary voltages in a transformer equals the ratio of the number of turns in their windings. A step-up transformer which increases voltage and decreases current, will have more turns in the secondary winding than in the primary winding. Similarly, a step-down transformer which decreases voltage and increases current, will have fewer turns in the secondary winding relative to the primary.
The relation between input primary voltage (Vp) and output secondary voltage (Vs) for a transformer can be determined by Faraday's law of induction. When a step-up transformer is designed to produce a higher secondary voltage from a lower primary voltage, the number of turns in the secondary coil (N) must be greater than that in the primary coil (Np).