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Sensitive Filament

Science Snack
Sensitive Filament
Use your breath to create a visible change in an electric circuit.
Sensitive Filament
Use your breath to create a visible change in an electric circuit.

The exposed filament from a 100-watt incandescent light bulb is wired in series with a flashlight bulb and a 9-volt battery. Blow on the filament and the flashlight bulb gets brighter.

Tools and Materials
  • 100-watt "rough service" incandescent light bulb (it’s a good idea to have more than one on hand for this activity)
    • "Replacement" bulbs for old incandescent bulbs (a 72-watt "replacement" bulb replaces an old 100-watt bulb) will not work—they have a filament, but it is enclosed in a halogen chamber and is not suitable for this activity
    • Do NOT try to use CFL, halogen, or LED bulbs
    • If you can find a traditional 100-watt incandescent bulb that isn't burned out, you can use it—see the Going Further section below for a brief discussion of the demise of traditional incandescent filament light bulbs and the role of "rough service" bulbs
  • Plastic or porcelain standard light bulb socket
  • Flashlight bulb, 2.25–2.5 volt (for two 1.5 volt cells), incandescent, E10 miniature screw base
  • Socket for flashlight bulb, E10 miniature screw base
  • 9-volt battery
  • Snap cap for 9-volt battery
  • 8-inch (20-centimeter) length of insulated wire, 20 or 22 gauge
  • Wire stripper
  • Mounting board, approximately 8 in x 12 in (20 cm x 30 cm); dimensions are not critical—the board just has to be big enough to mount the components
  • Two medium screws to mount large socket
  • Two small screws to mount small socket
  • Drill
  • Drill bits of appropriate size to make pilot holes for screws
  • Screwdriver
  • Plastic or paper bag big enough to hold the light bulb
  • 1 1/2-inch (4-cm) piece of double-sided foam mounting tape
  • Hammer
  • Needle-nose pliers
  • Safety goggles
  • Optional: work gloves

Connect the light sockets and battery:

  1. Connect one of the battery cap wire leads (color doesn't matter) to one of the contacts on the large light bulb socket and the other lead to one of the contacts on the small light bulb socket (strip the insulation from the ends of the leads if they're not already stripped).
  2. Strip the insulation off the ends of the piece of insulated wire. Connect one bare end to the remaining contact of the large socket and the other end to the remaining contact of the small socket.
  3. Place the two sockets on the mounting board and drill pilot holes into the board through the mounting holes of the sockets. Attach the sockets to the board with the screws.
  4. Connect the 9-volt battery to the snap cap. Use the foam mounting tape to hold the battery in place on the board. Alternatively, you can hammer two nails into the board on either side of the battery to hold it in place (bending them a little if necessary to make firm contact with the sides of the battery).

Prepare and connect the bulbs:

  1. Wear safety goggles for this part of the Assembly .
  2. You must remove the glass from the 100-watt light bulb without breaking the filament. Put the bulb into the paper bag and place it on a hard surface. Tap the bulb with the hammer with the smallest force necessary to break the glass (start with a tiny force and hit just a little harder each time until the glass finally breaks).
  3. Hold the bulb by its metal screw base and very carefully separate it from any large fragments of glass and then from the bag—the filament of the bulb is very fragile and can break if it bumps too hard against pieces of shattered glass. (Caution: You might consider wearing gloves while working with the broken glass and the bulb itself.) Use needle-nose pliers to crunch and remove any glass fragments remaining around the base of the bulb—be sure there are no sharp edges of glass left around the metal rim of the base, so you don't cut yourself when handling it.
  4. Screw the exposed bulb into its socket, being very careful not to break the filament in the process. (Note: If the filament breaks at any time, you need to throw the bulb away and start over with a new one.)
  5. Screw the flashlight bulb into its socket. The circuit is now complete, and the flashlight bulb should light up.
To Do and Notice

Blow gently on the exposed filament and notice any change in the brightness of the flashlight bulb.

Before you read the explanation below, try to figure out why blowing on the filament affects the brightness of the flashlight bulb.

Unscrew the flashlight bulb until it goes off, and let the exposed filament cool off for several seconds. Then screw the flashlight bulb in again and observe its brightness very closely for the first second or two after it lights up.

What’s Going On?

When you blow on the exposed filament, you actually cool it off because the air current carries away a fair amount of heat energy. As the temperature of the filament decreases, its electrical resistance decreases as well. This is because the atoms making up the filament vibrate less at lower temperatures, making collisions between the atoms and the electrons moving through the filament less likely. With fewer collisions, the electrons move more freely through the filament—in other words, they encounter less resistance.

Lowering the resistance of the exposed filament lowers the resistance of the complete circuit, allowing the flow of current in the circuit to increase. Since the flashlight bulb is part of the complete circuit, current through it also increases, making it glow more brightly.

When you screw in the flashlight bulb, its tiny filament heats and glows almost instantaneously, but it takes the large exposed filament a second or two to reach maximum temperature. For the short amount of time that the large exposed filament is relatively cool and has low resistance, the flashlight bulb glows very brightly; but once the exposed filament heats up and its resistance increases, the current in the complete circuit is reduced and the flashlight bulb dims.

When you turn on an incandescent lamp, the filament starts out at room temperature. While the filament is relatively cold it has a low resistance; it draws a large pulse of electric current at first, then settles down to a lower constant current. The initial burst of current can be ten times greater than the constant current. That's why incandescent light bulbs tend to burn out when they're first turned on: The initial large rush of current causes stress in the filament.

Going Further

Heating and Cooling
Use other means to change the temperature of the exposed filament (a match flame, a butane lighter, ice cubes, a hair dryer, etc.), and observe any change in the brightness of the flashlight bulb. Can you explain the changes that occurred? Caution: the exposed filament can get uncomfortably hot, and is very fragile.

Illegal Incandescents
Traditional incandescent light bulbs were notably inefficient, with more than 90% of their energy going to heat and less than 10% to light. The Energy Independence and Security Act of 2007 called for the phase-out of traditional incandescent light bulbs over a period of time. Production of traditional 100-watt incandescents stopped in 2012; 75-watt production stopped in 2013 and 60-watt and 40-watt production in 2014. In all cases, it was legal for merchants to sell and consumers to buy remaining inventories. Alternatives to traditional incandescent bulbs include CFL (compact fluorescent), halogen and LED (light emitting diode) bulbs.

An exception to the ban on incandescent bulbs is the rough service bulb. It differs from a traditional incandescent bulb by having extra support for the filament, so that vibration and rough usage is less likely to cause the filament to break. It is primarily intended for industrial use. Its energy consumption and lighting properties are generally comparable to the old bulbs, but it is notably more expensive.


We first saw this activity done by the Galileo Group of Japanese science teachers, and we would like to acknowledge their contribution.