In the given problem, we need to determine the voltage reading of an ideal voltmeter connected across an open switch in a DC circuit. Let's break down the scenario:

1. Consider the given circuit configuration with an open switch. The switch is wired such that one side connects to the supply voltage, say V_s, and the other side is connected to a load which ultimately leads to the opposite terminal of the supply.
2. Since the switch is open, there is no closed path for current to flow through the load circuit. Therefore, no current flows in the circuit.
3. An ideal voltmeter has infinite resistance, which means it does not allow any current to pass through it and measures the potential difference across its terminals.
4. With the switch being open, the voltmeter's terminals are effectively across the entire supply voltage, since one terminal is at the same potential as one side of the supply and the other terminal as the opposite side.
5. As no current flows, the potential difference between the points across the switch is equal to the supply voltage because those points directly connect each side of the supply when the circuit is conceptually complete without any current compensation.

Therefore, the voltmeter will read the full supply voltage because it measures the potential difference directly as there is no drop caused by current flow in the load or elsewhere in the circuit.

Hence, the correct answer is: The voltmeter will read the full supply voltage.