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Measuring Voltage and Current

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Measuring what we have learned

A meter lets us turn the ideas of voltage and current into actual numbers.

Reading the schematic: straight lines are wires; the unequal parallel lines are a cell/battery symbol; a zigzag is a resistor; a circle marked V is a voltmeter; a circle marked A is an ammeter. Labels beside the symbols tell us their values.
A V Ammeter: series Voltmeter: across the part

A useful first picture

A voltmeter is like comparing the height of two places: it looks across two points. An ammeter is like a turnstile counting everyone who passes: the current must go through it.

What is really happening?

A good voltmeter has a very high input resistance so it disturbs the circuit as little as practical. An ammeter has very low resistance so inserting it in series causes as little voltage drop as practical.

A little history

Early current measurements used magnetic effects produced by electric current. Instruments called galvanometers used a moving pointer to indicate small currents. Modern multimeters combine voltage, current, resistance, and other measurements in one instrument, often with a digital display.

Worked example

V

Question: You want to know the voltage across a lamp. Where do the two meter leads go?

Answer: One lead goes to each side of the lamp. The voltmeter is connected across the lamp.

Try these yourself

A voltmeter is normally connected: V
Voltage is a difference between two points, so a voltmeter is connected across those points.
An ammeter is normally connected: A
To measure current, the circuit path is opened and the ammeter is inserted in series.
Why is putting an ammeter directly across a battery dangerous? A
An ammeter is designed to have very low resistance. Across a source it can draw excessive current, often blowing the meter fuse and possibly causing damage.

A little deeper

Real meters are not perfect. A voltmeter draws a small amount of current, and an ammeter introduces a small amount of resistance. In most ordinary troubleshooting those effects are small, but in sensitive circuits they can matter.

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