Posted on 11 December 2011 by EliRabett

RW Wood was perhaps the greatest experimental physicist of his time.  Eli, old Rabett that he is did not know him, but he took classes from several who did and others who studied under him.  Wood was a practical man which gave his experimental work a particular clarity.

To the public, as far as he is known, he is famous for two experiments.  The first was his debunking of Blondiot's N-Rays, the second, his experiment purporting to show that the greenhouse effect did not happen as advertised. The Weasel has kindly posted the text of the later and Eli has had more than a passing interest in the same.  About a year ago he attempted a preliminary test of Wood's work, which was instructive, but not conclusive.  Rabett Labs was tooling up for a second improved try, when he was invited to submit something to Skeptical Science (cross-posted there and here, whichever there is there and here is here) and DeWitt Payne also wrote him to describe his attempt.

Let us briefly reprise what Wood did

To test the matter I constructed two enclosures of dead black cardboard, one covered with a glass plate, the other with a plate of rock-salt of equal thickness. The bulb of a themometer was inserted in each enclosure and the whole packed in cotton, with the exception of the transparent plates which were exposed. When exposed to sunlight the temperature rose gradually to 65 oC., the enclosure covered with the salt plate keeping a little ahead of the other, owing to the fact that it transmitted the longer waves from the sun, which were stopped by the glass. In order to eliminate this action the sunlight was first passed through a glass plate.  

There was now scarcely a difference of one degree between the temperatures of the two enclosure

There are several things wrong with this, well, not so much wrong, as not useful and to be complete, equally wrong with many so called demonstrations of the greenhouse effect that can be seen on You Tube.  To understand this the bunnies recommend a shortened version of one of Eli's simple greenhouse effect explanations, but boiled down

1.  Greenhouse gases absorption blocks some thermal radiation from the surface reaching space.

2.  Temperature decreases with altitude.

3.  The absorbed radiation and convective heating moves energy up from the surface

4.  The rate of emission to space in the blocked regions occurs from colder altitudes.

5.  Increased greenhouse gas concentrations mean that the altitude at which emission in the blocked regions occurs is higher and colder.

6.  Therefore energy loss is slower and the surface has to warm in order to restore radiative balance.

Now, let us look at Wood's experiment, first being McIntyre picky.  For one thing, the sensitivity of the thermometer is not impressive.  Also, in the case of the rock salt window a significant part of the IR from the black surface inside the cell is absorbed and for a third heating with sunlight brings additional problems.


the difference up to about 10 microns is due to reflections from the surface, and does not represent absorption.  Still, the major problem(s) are that

(a) the walls of the cell are excellent absorbers of thermal energy and light and (b) most importantly, there almost certainly was not a temperature difference between the top and the bottom of the cell.  We can get a hint of what this means by looking at some MODTRAN spectra.  In the first, looking down from 20 km

while if we look down from 1 km what the bunnies see is

because the temperature at the top and the bottom of the path are about the same and emission and absorption of the greenhouse gases is balanced.  In the case of the glass window, the IR emission came from the surface  of the window.  In the case of the rock salt, from the black walls which  were at the same temperature.  For the purpose of this analysis, the  emissivity of the two materials can be taken as equal, at least for the  sensitivity of the thermometer.

This tells us a few things about how to do a new and improved Wood experiment.  First, the inside of the cell should be silvered, to reflect IR light.  Second, heating should be done at the bottom of the cell.  Third, the top should be colder than the bottom.  Fourth, a much more sensitive thermometer is needed.  Fifth, KBr would be a better window than NaCl, but that is a quibble.