Continuity testers are electrical test instruments that are used to determine if an electrical circuit exists between two points. They consist of an indicator that is connected in series with a power source (usually a battery), and two test leads or probes. The indicator is activated if the probes detect a complete circuit between the leads. Some continuity testers have visual indicators such as light emitting diodes (LEDs). Others feature audible buzzers. For high-resistance circuits and applications which involve sensitive electronic components, low-voltage and low-current continuity testers should be used. Typically, these devices contain an operational amplifier or op-amp.