Electrical Circuits, Voltmeters And Ammeters

                    Figure 1.2: (a) a closed circuit where current (I) can flow; (b) an open circuit where no current can flow.

•  The path around a circuit must be unbroken for an EMF to push electrons around it, such a circuit is said to be closed. If a switch is included in the circuit it can be used to break the path and produce an open circuit where no electrons can flow (figure 1.2a and 1.2b).
• In a high voltage circuit, current may arc across a small gap causing considerable damage. Most switches will arc briefly when opened however, as long as the current doesn't jump the gap for too long, it is not a problem. 
• Voltmeters measure a potential difference between two points and so must be connected in parallel. If you take a voltage reading across a supply in a open circuit you will be measuring the supply's EMF, you can only measure voltage drops across a load (e.g. a piece of equipment) if a current is flowing. Figure 1.3 shows the placement of voltmeters in a circuit. 
• Ammeters measure the current flowing through a point in a circuit, therefore they are connected in series. Figure 1.4 shows the placement of a voltmeter in series.