Measure a Flashlight

Measure for measure


Use a meter to measure the voltages and currents in a simple electrical circuit.



Connect two wires to the battery.

See Making Battery Holders

To Do and Notice

Measure the voltage across the battery.
Set the voltmeter to a scale which will read the approximate 1.5 volts of the battery. Connect the common lead of the meter to the negative terminal of the battery (usually a black wire) and the positive lead to the positive terminal (usually red). Notice the voltage, it may be +1.55 volts.

Do not measure the current yet.

Connect one battery to the bulb.
The light will glow dimly.
Measure the voltage across the battery in the same way as above. Notice that this is the same voltage that is across the bulb.
Change the scale on the multimeter to read amperes. Break the circuit and insert the meter to read the current through the meter. See
Meters, Current

Repeat the above measurements using two batteries.

What’s Going On?

A Flashlight is a simple circuit. One or more batteries, one bulb and some wires.

The electric current in the same everywhere in the circuit.

The batteries create a voltage across the bulb that drives the electric current through the bulb.

Math Root

We can find the resistance of the bulb by applying ohms law to the bulb.

Ohm's law is V = IR

Where V is voltage, in volts
I is current, in amps
and R is resistance in ohms

so R = V/I

and for my bulb and batteries I measured

with one battery R = 1.6 V/.08 A = 20 W

with two batteries R = 3.2 V/0.1 A = 32 W

The bulb has resistance but its resistance changes with the current that flows through the bulb.

Higher currents cause the filament of the bulb to get hotter and to emit more light. The hotter tungsten filament causes the tungsten atoms to vibrate more violently. Electrons moving through the wire collide with the tungsten atoms more often when the tungsten atoms are moving. This makes hotter wires have higher resistance.


Electric meters often have internal fuses which protect them. If too much current flows through the meter you will blow the fuse and the meter will stop working. It is an easy matter to replace the fuse.

Return to Day 15

Scientific Explorations with Paul Doherty

© 1999

30 May 2000