What does a voltmeter specifically measure in an electrochemical cell?

A voltmeter measures the electromotive force (EMF) between the electrodes in an electrochemical cell. In this context, the EMF represents the potential difference that drives the flow of electrons in the circuit, effectively indicating how much energy is available per unit of charge. This measurement is crucial because it provides insights into the cell's ability to perform work and is a key parameter in assessing the cell's overall performance.

Understanding the EMF helps in characterizing the cell, including its efficiency and what might happen during a chemical reaction within it. The EMF can vary depending on the concentration of reactants and products and the overall reaction conditions, making the measurement of EMF a vital part of analyzing electrochemical cells.

The other options refer to aspects that, while important in the context of electrochemical cells, are not directly measurable by a voltmeter. Temperature could affect reactions but is not what a voltmeter measures; internal resistance relates to how much the cell resists current flow, which would typically be measured using different methods; and current flow is measured with an ammeter, not a voltmeter.