Voltmeter

The voltmeter is designed to be connected directly across the source of powers a voltmeter being used to test the voltage of a lot­tery. The leads of the meter are connected directly across the source of voltage. A voltmeter can be connected directly across the power source because it has a very high resistance connected in series with the meter move­ment. The industrial standard for a voltmeter is 20,000 ohms per volt for DC and 5000 ohms per volt for AC. Assume the voltmeter is an AC meter and has a full-scale range of 300 volts. The meter circuit (meter plus resistor) would, therefore, have a resistance of 1,500,000 ohms (300 V X 5000 Ω per volt = 1,500,000 Ω.

Measurements of de current and voltage are made with direct-reading (analog) or digital meters. A direct-reading meter has an indicating pointer whose angular deflection depends on the magnitude of the variable it is measuring. A digital meter displays a set of digits indicating the measured variable value. To measure a voltage or current, a meter is connected to a circuit, using terminals called probes. These probes are color coded to indicate the reference direction of the variable being measured. Frequently, meter probes arc colored red and black. An ideal voltmeter measures the voltage from the red to the black probe. The red terminal is the positive terminal, and the black terminal is the negative terminal.  An ideal ammeter measures the current flowing via its terminals, as shown in and has zero voltage, V_m, across its terminals. An ideal voltmeter measures the voltage across its terminals and has terminal current, I_m, equal to zero. Practical measuring instruments only approximate the ideal conditions. For a practical ammeter, the voltage across its terminals is usually negligibly small. Similarly, the current into a voltmeter is usually negligible. Ideal voltmeters act like open circuits, and ideal ammeters act like short circuits. In other words, the model of an ideal voltmeter is an open circuit, and the model of an ideal ammeter is a short circuit.

Questionnaire:

• What is voltmeter? Explain.