1. This a very impressive looking rule. It is 18" long and 3 ½" wide and weighs 600 grams (1lb. 6 oz). It is designed to study black body radiation. A black body being one that radiates energy at 100% efficiency. Some idea of its size can be gained by comparing the images here with those of a normal 10"rule since they are to scale scale. It was copyrighted by the UK's Admiralty Research Laboratory.
2. The idea that bodies which emit light also emit heat was first explored by the English astronomer Herschel in 1800 who, when he moved a blackened thermometer through the light separated into its constituent colours by a prism, found that the heating effect increased as he moved toward the red end of the spectrum and continued even past the red into what we now call "infra-red". Whilst initially this form of radiation appeared to be well understood it was found that the classic equations, due to Maxwell, broke down at the opposite end of the spectrum - this was called the ultraviolet catastrophe. It was Planck who proposed the solution - quantum theory. He showed that the pattern of radiation could be explained if radiation did not vary as a continuous function but as discrete quanta of energy.
3. The rule is designed to calculate the energy and photons emitted at different temperatures and wavelengths. The scales (circle is stock and square is slide) are:
- g - Ql max photons sec.-1 cm.-2 cm.-1Dl
- f - Q photons sec.-1 cm.-2
- e - wavelength l micronl
On the slide for "Energy":
- h - Hl / Hl max
- j - Multiplication scale for use with Hl max (b) only
- i - Hl / Hl max
- l - Hl-inf / H = I - Hl-inf / H
- k - Multiplication scale for use with H (a) only
- m - H0-l / H
- d - T degree absolute
- c - t degree centigrade
- b - Hl max watts cm.-2 cm.-1Dl
- a - H watt cm.-2
On the reverse of the slide for Photons:
- n - Ql / Ql max
- p - Multiplication scale for use with Ql max (g) only
- o - Ql / Ql max
- r - Ql-inf / Q = I - Ql-inf / Q
- q - Multiplication scale for use with Q (f) only
- s - Q0-l / Q
4. The setting of the cursor shows the rule set for temperature of 900° K. At this temperature the total radiant energy is 3.7 watt cm.-2 (scale a). It reaches its maximum spectral black body energy of 7,600 watts cm.-2 cm.-1Dl (b) at a wavelength of 3.22 microns (e). Using the wavelength scale and scale l it possible to calculated the total energy between any pair of wavelengths; for a value of 3 microns the proportion of energy from 0 to that wavelength is 0.205 and for 5 microns the equivalent figure is 0.565. Multiplying the difference of these two proportions by the value of total radiant energy 3.7 watt cm-2 gives a value of 1.33 watt cm-2 .
5. The scale h enables the shape of the curve relating spectral energy to wavelength to be determined. This scale has an interesting feature shown in the detailed image below. Where the scale becomes very compressed, rather than squeezing several lines into a short space a graphical representation is used. In the example show the hairline is at the value of 2 x 10-8.
6. The rule came in a teak wood box and has very basic instructions inside the lid of the box.
Slide reversed
Detail - slide
