Portrait of Thomas Edison

Adapted from ⁠an image by Louis Bachrach, Bachrach Studios, restored by Michel Vuijlsteke (public domain), via Wikimedia Commons.

1847–1931

Thomas Edison

American inventor of the phonograph and a practical electric lamp, with more than a thousand US patents.

Element

6CCarbon
1879

At Menlo Park, Thomas Edison's team lit a lamp with a filament of carbonised cotton thread in a vacuum, and it burned for over 13 hours. It was the first lamp with real commercial promise.

Explore carbon

Edison's first lamps glowed with carbon filaments; tungsten took over in the 1900s. Turn up the current on a filament.

Light bulb filament

Turn up the current on a light bulb

An old-style light bulb is just a coil of tungsten wire. Push a current through it and the wire heats up until it glows. Turn up the current and watch the colour change, then see how close the filament gets to melting.

Filament temperature20 °C293 K · No visible glow
Current
0.00 A
Resistance
57 Ω
Visible light
almost none

Switched off. Turn up the current to heat the filament.

Cold, the filament’s resistance is only about 57 Ω; glowing, it’s about 882 Ω. So for a split second after you switch on, around 15 times the normal current rushes in, which is why bulbs usually fail at the moment you switch them on.

Why tungsten?

  • Gold1,064 °C
  • Copper1,085 °C
  • Iron1,538 °C
  • Platinum1,768 °C
  • Tantalum3,017 °C
  • Osmium3,033 °C
  • Tungsten3,422 °C

The bars show each metal’s melting point; the orange line is the filament now. A normal bulb at 2,427 °C would have melted gold, copper, iron and platinum long ago. Only a handful of metals survive it, and tungsten melts hottest of all.

Where the energy goesHow much energy a glowing object gives out at each wavelength of light, in nanometres (nm). Visible light runs from about 380 nm (violet) to 750 nm (red); longer wavelengths are infrared, which you feel as heat. The curve is scaled to its own peak, so it keeps its height as the filament heats up.
VisibleInfrared (heat)5001,0001,5002,0002,500 nm

Even at room temperature the filament gives out a faint glow, but it’s all far infrared: the peak is off the right of the chart.

A century of filaments

  • Edison’s carbon, 1879

    The first practical bulbs used carbon threads. Carbon doesn’t melt, but it slowly boils away, blackening the glass, so the filaments had to run cooler and redder.

  • Osmium, tantalum, tungsten

    Osmium (1898) and tantalum (1902) came next. Tungsten arrived in 1904, but it was brittle until William Coolidge found a way to draw it into fine wire; by 1911 it was in bulbs everywhere.

  • Why they were phased out

    Even at its best, a glowing wire turns only a few percent of its power into light. LEDs give the same light for a fraction of the energy, so many countries have banned most filament bulbs.

A 60 W, 230 V bulb, calibrated to a typical 2,700 K at its rated current. The temperature comes from balancing the electrical heating against the heat radiated, using tungsten’s measured resistivity (Desai et al., 1984); it ignores the heat lost through the supports. Real filaments radiate a little less than an ideal blackbody, and because our eyes are most sensitive to green, a real 60 W bulb’s efficiency measured by brightness is only about 2%. Melting points from the element data. Bulb history from Wikipedia.

Keep exploring

Where the dates and stories on this page come from; each event on the timeline links to its own source.