Sci: Measuring light

 

Joseph Wright of Derby, ‘A Philosopher Giving a Lecture at the Orrery’, a chiaroscuro painting. A lantern plays the part of the sun. I explain chiaroscuro at the end.

This leads to two really neat kids-you-can-make-this-at-home gadgets. First, though, we need some background. The Dark Ages really were dark ages, but the unlit ages went on longer than the Dark Ages. It seems hard to imagine now, but the normal 18th century night, even in a rich household, was like this chiaroscuro painting.

So how do you get a really bright light source? Tallow and wax candles are a start, but they would hardly be bright enough for what we expect today. Until the early nineteenth century, when electric arcs were used, there were no lights as bright as daylight. Even in 1846, when the Paris Opera was lit by arc lamps, the power came from Robert Bunsen’s electric cells that quickly lost their power. (Remember Bunsen: he comes up again, soon.)

Before 1880, bright lighting was all about finding efficient oil lamps. Efficiency is important, and if you want a lot of light in your house, and you are burning hydrocarbons, especially large molecule hydrocarbons, you need a big clean flame, free of smoke, and that means getting enough oxygen into the flame. This brings us to Aimé Argand, who invented a lamp in 1782.

The Argand burner design can work with either oil or gas. In oil Argand lamps, the fuel is burned on a hollow cylindrical wick, with air being introduced inside the wick, to give a large area of contact between the air and the fuel. The gas Argand lamp used a cylindrical flame, but there was no wick. In the hollow cylindrical flame, all of the fuel is burnt in a nice bright flame, and there is no messy smoke or soot. The flames of the Argand lamps were so bright that some were rated as high as 1200 candlepower.

Lighting mattered because 19^th century industrialists were tying up so much money in machinery that it made good sense to work a second shift at night. Then they got twice the return, just for providing wages, fuel for the steam engines, and a bit of oil for the efficient Argand lamps.

These days, we are used to bright lights, and the Argand lamp seems unimpressive, but in its day (which lasted for about a hundred years), it had a remarkable social effect. Later forms were fitted with governors to even out pressure variations, and these sold in huge numbers. Eventually though, the Argand lamp reached the end of its life.

Cheap mineral oil was available as a fuel, and gas burners with incandescent mantles would soon enter the market. Even so, some of the early mantle lamps used Argand burners, but it was a desperate last effort. All over the world, the Argand lamps were going out, because people could accurately compare rival light sources and choose the best.

And that brings us back to Bunsen…

The Bunsen grease-spot photometer

Artificial light was important, and there was money to be made if you could compare the brightness of different sources. Brown paper is less common than it used to be once, but if you have used it much, you will know that the paper shows grease marks really well:

Depending on where the light is, the grease shows up as dark or light.

I was lucky in my first science teacher, Andy (‘Penguin’) Watson. He had explored Antarctica in the early 20^th century, returned to become a high school principal, and after he retired, he came back as a ‘retread’, to teach what he loved, which was science.

His teachers would have got their training in Queen Victoria’s time, when the notion of using string and sealing wax to make equipment was common. Back then, the crazy inventions of Heath Robinson, Rube Goldberg and Storm P (chapter 8) didn’t seem so crazy. Now you can see, perhaps, why I learned at school about the invention Robert Bunsen wasn’t famous for, though unlike the Bunsen burner, Robert Bunsen really did invent the photometer that carries his name, and ‘Penguin’ knew all about it.

Two versions of the Bunsen grease-spot photometer.

The photometer compares the intensities of two light sources. It is just a piece of brown paper with a greasy spot on it. When the light on the far side is brighter than the near-side light, the grease spot looks lighter than the surrounding paper. When the light on the far side is weaker, the spot appears dark, but if the two light sources are balanced, the spot disappears.

The photometer is easy to make: just use ordinary brown paper or even the white ‘butcher’s paper’ that comes around your fish and chips, while butter, margarine or olive oil can also make the grease spot. If you do not know what the inverse square law is, now would be a good time to look into it: see the notes for this chapter.

No jokes about greased lightning, though, because even I would not sink to that level. You can use this photometer to calibrate the sun’s brightness in the early morning or the late afternoon. You might even use it to test when official daylight starts: in the 18th century Royal Navy, daylight was by tradition the time during which you could “see a grey goose at a mile”.

Islamic tradition identifies dawn during Ramadan, the month of fasting, by an imam holding two threads at arm’s length, one black and one white, to see if they can be distinguished by the available natural light. Could a “standard candle” and a Bunsen photometer be used to assess the change from night to day in a similar way?

The Rumford shadow photometer

The Rumford photometer uses a more subjective comparison of shadows, cast by two lights onto a screen.

Once again, you need to keep in mind the inverse square law and do a bit of calculation before you can find a simple comparison of the power of two lights.

A photometer like Rumford’s could be used to compare the strengths of candles and dips made from oil of different quality, or to assess the effect of reflectors, placed behind a flame to increase its apparent brightness.

Play with it!

Some notes

The word chiaroscuro comes from two Italian words, chiaro, meaning light and oscuro, meaning dark. It refers to the dramatic effects painters can get from contrasting light and dark.

Sealing wax: when a colleague found some red sticks of wax in the stationery cupboard, he thought they were crayons, but they had a square cross-section and they didn’t mark paper very well. He knew I was interested in old things so asked me if I knew what they were for. He was very lucky, because my father used to use sealing wax, and I recognised it. There are probably only a handful of Australians who have used it for its proper purpose.

Sealing wax is a hard type of wax that can be melted to seal things like important documents, or things that need to be very secure. If you look on the web, you can find places where you can buy sealing wax, and you can get ideas for amusing art activities. Be warned: you will be using flames and hot wax that burns the skin. Suggested age: 15+!

Photometers and the inverse square law

No, I won’t explain this diagram. Work it out.

Here is a simple law of physics that explains how certain forces or intensities differ with distance. The force of gravity is affected by this law, and so too is the intensity of light from a point source. It is usually credited to Sir Isaac Newton, although Robert Hooke felt he also deserved a share of the credit.

In its simplest form, the law states that the strength of a force or a radiation is reduced to one quarter of what it was if you double the distance, or to one ninth, if you triple the distance. In other words, given the intensity at one unit of length from the source, then at x units, the intensity will be 1/x². Of course, if the distance is halved, the force or intensity will be quadrupled.

There is one small caution that needs to be sounded here. In the case of the earth’s gravitational force, the starting place is the centre of the earth, as the force of gravity from a spherical body acts like an extremely dense mass, located at the centre of the sphere. So a spacecraft which lifts up to one earth radius (around 6400 km) above sea level will experience one quarter of the gravitational pull that we feel on the earth’s surface at sea level.

And who invented the Bunsen burner? Peter Desaga perfected Michael Faraday’s design. Robert Bunsen just made the design popular.

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