In 1878, 44-year-old American astronomer and physicist Samuel Pierpont Langley (1834Constant–1906) invented the Bolometer—an ultra-sensitive instrument designed to measure radiant thermal energy across infrared and solar spectra.

Langley presented the device to the American Academy of Arts and Sciences in 1880, achieving a temperature sensitivity of $1/100,000^{\text{th}}$ of a degree Celsius ($10^{-5}\,\text{K}$).

Operating Principle of the Bolometer

The bolometer measures thermal radiation through temperature-dependent electrical resistance changes in thin platinum foil:

1. Wheatstone Bridge Sensor

Langley constructed two ultra-thin blackened platinum strips ($1\,\mu\text{m}$ thick) connected into a Wheatstone bridge circuit:

  • One strip was exposed to incident infrared radiation, while the second reference strip remained shielded.
  • Radiant heat absorption elevated the exposed strip’s temperature $\Delta T$, altering its electrical resistance:

$$\Delta R = R_0 \cdot \alpha \cdot \Delta T$$

Where $\alpha$ is the temperature coefficient of resistance.

2. Infrared Mapping

Using the bolometer, Langley mapped the far-infrared solar spectrum out to $5.3\,\mu\text{m}$, discovering hundreds of previously invisible infrared absorption lines.

Critical Role in Quantum Physics Discovery

Langley’s bolometer provided Heinrich Rubens, Ferdinand Kurlbaum, and Otto Lummer at the PTR in Berlin with the precise experimental sensor needed to measure blackbody radiation curves at long infrared wavelengths ($12-18\,\mu\text{m}$).

Their precision bolometer measurements revealed the failure of Rayleigh-Jeans classical theory, directly prompting Max Planck to formulate the Quantum Hypothesis in December 1900.

In modern astrophysics and quantum computing, Transition Edge Sensor (TES) Cryogenic Bolometers operating at $100\,\text{mK}$ detect single microwave and cosmic microwave background (CMB) photons.

Key Takeaways

  • Year: 1878
  • Key Figure: Samuel Langley (American Astronomer)
  • Core Discovery: Invented the Bolometer, capable of measuring radiant heat changes of $10^{-5}\,\text{K}$.
  • Infrared Pioneer: Extended solar spectrum mapping far into the infrared wavelength regime.
  • Quantum Relevance: Enabled precision blackbody radiation curve measurements that led to Max Planck’s quantum theory ($E = h\nu$).