Analog voltmeters are instruments that measure voltage or voltage drop in a circuit. They display values on a dial, usually with a needle or moving pointer. Analog voltmeters are used to locate excessive resistance that may indicate an open circuit or ground. They are also used to identify low voltage or voltage drops that may indicate a poor connection. Analog voltmeters are connected in parallel (and never in series) with the circuit being tested so that the meter can tap a small amount of current. The positive lead is connected to the circuit's positive side and the negative lead is connected to the circuit's ground. An ammeter is a measuring instrument used to measure the flow of electric current in a circuit. Electric currents are measured in amperes. Modern ammeter designs are non-mechanical, or digital, and use an analog to digital converter to measure the voltage across the shunt resistor. Frequency meters are instruments that provide analog outputs as frequencies that vary as a function of the applied input. They display values on a dial, usually with a needle or moving pointer. Frequency meters measure repetitions per unit of time, usually per second, of a complete electromagnetic waveform.