# What was Planck's Black Body?

What was Planck's Black Body. I mean physically what was it, a piece of iron? I have searched the web without success. My daughter and I are going to find the constant using LED's. Here's the board I created. I'd like to know what Planck actually used to find the constant. I know LEDs were not available to him. Any insight to his thoughts and processes would help.

• I'm not at all sure that Planck did any experiments on the matter. He was simply working out a functional form to link the two published successful asymptotic expressions. I'd guess that most of the tools used by experimenters at the time were cavities of one kind or another. Whether they would have used stone or metal I wouldn't know. – dmckee Oct 1 '16 at 2:16

## 1 Answer

There is an answer here about how Planck "measured" the constant in the black body formula:

By positing that electromagnetic radiation at high frequencies fell off exponentially Planck's law at high energies fundamentally introduced a new constant. Planck could measure Planck's constant immediately by deriving Wien's displacement law which gave $$h=xkT\lambda_\textrm{max}/c$$ where x is the solution to the transcendental equation $$e^{-x}=(1−x/5)$$ which is $x=4.965.$ He knew pairs of $T$ and $λ_\textrm{max}$ and could immediately measure this constant.

Thus the original data is in Wien's displacement law ,and Wien's constant which was also derived from thermodynamic arguments, $b$ comes from existing data.

By the 1890s, various experimental and theoretical attempts had been made to determine its spectral energy distribution - the curve displaying how much radiant energy is emitted at different frequencies for a given temperature of the black body.

So, as in most of physics, theoretical constants are dependent on many experiments, so not one set up was used in the original derivation of $h.$ It is a nested set. Planck's black body was many bodies, rather the mathematical idealization of many bodies in Wien's law.