Site of the first ever cosmic ray experiment

Wilson's first experiments on controlled expansion of moist air revealed that a few ions per cubic cm are produced each second, in normal air at sea level. It follows immediately that air is a weak conductor of electricity, and he and others demonstrated this to be the case in 1900. What natural agent continually produces these ions? Wilson's 1901 suggestion was boldly imaginative: "...radiation from outside our atmosphere, possibly radiation like Roentgen rays or like cathode rays, but of enormously greater penetrating power." To test this idea he took a portable form of electroscope into the tunnel of the Caledonian Railway near Peebles (shown here). Wisely he did this "at night, after the traffic had ceased." His thinking was that the thickness of earth above the tunnel would absorb this cosmic radiation, so that fewer ions would be produced and the air there would be a poorer conductor of electricity. In tunnels and mines, however, you are surrounded by rocks whose own, natural radioactivity (at a normal, low, safe level) could hide any reduction in radiation of cosmic origin. Wilson's experiment was flawed for his purpose. Nonetheless it marks the start of the experimental study of cosmic radiation. We know now that cosmic radiation accounts for something like 10% of the ionisation of air at sea level. The remainder is due to naturally occuring, trace quantities of radioactive substances.

Dominic knows those parts of the world very well. A couple of years ago he and I spent a day in the Pentland country visiting, in particular, this site of the first ever cosmic ray experiment. When we visited it was still accessible and clearly used from time to time for a variety of non-official purposes. I believe the railway line here closed in 1954, however, even before Beeching. We hadn't thought to bring a torch so didn't go very far inside, and you don't have to go far before it gets quite dark and tripping on an empty Buckfast bottle becomes a real possibility. This Geograph photo, taken in July 2008 at the tunnel's far end, seems to show some tidying up operation in progress so perhaps it will be closed off to protect people from themselves. In the meantime it does appear that anybody with the interest can visit this Scottish scientific landmark, unmarked and unheralded though it is.