On an energized, intact circuit, you will read voltage across an OPEN switch when testing with a voltmeter. However, you probably won't read significant voltage across a CLOSED switch.

Both sides of a closed switch are electrically identical (or at least very close). Therefore, there should be no movement of electrons between the leads of your voltmeter.

I have seen many new apprentices get confused when they measure across the points of an energized (closed) contactor or between two energized low voltage circuits, and they measure 0 volts.

Voltage measurement is always a measurement of the potential difference between two points. It's not simply a measurement of how much “electricity” can be measured at one point. (Current or amperage represents the actual electrons moving through a circuit. It's so easy to learn that I even taught my sons about basic electrical theory.)

So, just remember:

Across a closed switch = 0 volts (or if it does display voltage, it is the voltage drop across the switch)

Across an open switch = applied voltage

—Bryan