When using the "three lamp" method, the two alternators are in phase when all of the lamps are?

The correct answer is that all of the lamps are off when the two alternators are in phase using the "three lamp" method. This method is designed to compare the phase relationship between two alternating current (AC) sources, typically alternators or generators.

In this scenario, when the two alternators are perfectly in phase, their voltages reach their peaks and zero points simultaneously, resulting in no voltage differential across the lamps. Consequently, the lamps do not light up, which is why they are all off. The fundamental principle here involves the phase relationship between the two sources; if they are in phase, there is no current flowing through the lamps because there is no voltage difference to drive the current.

The other options—flickering, on, or dimmed—indicate varying degrees of voltage or current flow, which imply that there is a phase difference between the two alternators. Such conditions would mean either a constructive or destructive interference of the voltages, leading to the observed lamp conditions. Thus, for the lamps to be off, it confirms that both alternators are perfectly synchronized and in phase.