In set-up with two half-cells and a voltmeter, the potential difference is measured using the voltmeter. But voltmeter has high resistance,which i know is still able to measure potential difference,but wouldnt it prevent electrons from flowing What i meant was,the use of voltmeter creates a 'open circuit' which prevents electron flow and hence reaction in the two half cell will not take place? But my teacher says that reaction takes place in both half cell,which meant that there is electron transfer and redox?? Does this have to do with some physics regarding voltmeter??