In principle, a voltmeter measures the electrostatic potential, the physical quantity that we will be talking about in these sections of the book. But you cannot use a voltmeter to measure the electrostatic potential due to a collection of charges that are just sitting around in the lab, since that is not the way that voltmeters actually work in practice. Normally, you attach the two probes of a voltmeter to different places in a circuit. The voltmeter contains a really large resistor and measures the current flowing across that resistor. The scale on the voltmeter automatically converts the measured current to a voltage reading using $V=IR$. If, instead, you try to measure the the potential difference between two points in air (or, even worse, in vacuum), the resistance of the air between two voltmeter probes is even larger than the resistance in the voltmeter, exceeding its design parameters.