If an SPST switch is closed and 120 VAC is applied, what should the multimeter read when placed across the switch?

When an SPST (Single Pole Single Throw) switch is closed and 120 VAC is applied, this means that the switch is currently in the "on" position, allowing electrical current to flow through it. When you use a multimeter to measure the voltage across the closed switch, it should read 0 Volts.

This is because the switch does not introduce any significant resistance or voltage drop when it is closed; it is essentially a direct connection in the circuit. Since the multimeter measures the potential difference across the two terminals of the switch and both terminals are at the same electrical potential (connected), the reading is zero.

In contrast, if the switch were open, you would see the full 120 VAC across it. However, with the switch closed, there is no potential difference to measure, resulting in a reading of 0 Volts. This concept aligns with basic electrical principles regarding closed circuits and how voltage is measured.