Voltmeters

Current, as we have seen, consists of a flow of charge carriers. Voltage, or electromotive force (EMF), or potential difference, is the “pressure” that makes current possible. Given a circuit whose resistance is constant, the current that flows in the circuit is directly proportional to the voltage placed across it. Early electrical experimenters recognized that an ammeter could be used to measure voltage, because an ammeter is a form of constant-resistance circuit. If you connect an ammeter directly across a source of voltage such as a battery, the meter needle deflects. In fact, a milliammeter needle will probably be “pinned” if you do this with it, and a microammeter might well be wrecked by the force of the needle striking the pin at the top of the scale. For this reason, you should never connect milliammeters or microammeters directly across voltage sources. An ammeter, perhaps with a range of 0 to 10 A, might not deflect to full scale if it is placed across a battery, but it’s still a bad idea to do this, because it will rapidly drain the battery. Some batteries, such as automotive lead-acid cells, can explode under these conditions.

Ammeters have low internal resistance. They are designed that way deliberately. They are meant to be connected in series with other parts of a circuit, not right across a power supply. But if you place a large resistor in series with an ammeter, and then connect the ammeter across a battery or other type of power supply, you no longer have a short circuit. The ammeter will give an indication that is directly proportional to the voltage of the supply. The smaller the full-scale reading of the ammeter, the larger the resistance that is needed to get a meaningful indication on the meter. Using a microammeter and a very large value of resistance in series, a voltmeter can be devised that will draw only a little current from the source.

A voltmeter can be made to have various ranges for the full-scale reading, by switching different values of resistance in series with the microammeter (following figure). The internal resistance of the meter is large because the values of the resistors are large. The greater the supply voltage, the larger the internal resistance of the meter, because the necessary series resistance increases as the voltage increases.

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A simple circuit using a microammeter (μA) to measure dc voltage.

A voltmeter should have high internal resistance, and the higher the better! The reason for this is that you don’t want the meter to draw much current from the power source. This current should go, as much as possible, toward operating whatever circuit is hooked up to the power supply, and
not into getting a reading of the voltage. Also, you might not want, or need, to have the voltmeter constantly connected in the circuit; you might need the voltmeter for testing many different circuits. You don’t want the behavior of a circuit to be affected the instant you connect the voltmeter to the supply. The less current a voltmeter draws, the less it affects the behavior of anything that is working from the power supply.

A completely different type of voltmeter uses the effect of electrostatic deflection, rather than electromagnetic deflection. Remember that electric fields produce forces, just as do magnetic fields. Therefore, a pair of plates attract or repel each other if they are charged. The electrostatic voltmeter takes advantage of the attractive force between two plates having opposite electric charge, or having a large potential difference. Following figure is a simplified drawing of the mechanics of an electrostatic voltmeter. It draws almost no current from the power supply. The only thing between the plates is air, and air is a nearly perfect insulator. The electrostatic meter can indicate ac voltage as well as dc voltage. The construction tends to be fragile, however, and mechanical vibration can influence the reading.

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A functional drawing of an electrostatic voltmeter movement