When I first started writing the Classic Kit column I knew that the McLeod gauge was something that needed writing about. I mentioned it in passing in an earlier column on Sprengel but at the time I couldn’t find out much about McLeod’s career, not could I find a portrait of the man.
The gauge itself is a very cunning device for measuring low pressures (or high vacuums, if you prefer). From the moment that Geissler and Töpler had made their piston pumps, and then Sprengel had come up with the more “automated” mercury dropping pump, there was a real need to find out how low the pressure was; especially since there was growing excitement about the strange glows that appeared when electric currents were passed through rarefied gases – such as in the Geissler or Plücker tubes (lots of nice pics and stories here) that became such desirable curiosities for fashionable gentlemen with a recreational interest in science.
An ordinary manometer, however, can only be read to a precision of plus or minus a millimeter or so. Add in a travelling microscope (they used to be called “cathetometers”) equipped with a Vernier scale and you might be able to squeeze a significant figure or more. But that just wasn’t good enough.
Herbert McLeod solved the problem. He did so simply by using Boyle’s Law
pV = constant
and combining it with the Geissler/Töpler mercury piston method. To measure the pressure you simply raised a mercury reservoir to isolate a slug of residual gas of fixed volume, and then raising it further compressed it into a fine capillary. The capillary could be calibrated in units of pressure and McLeod gauges could be used to measure pressures down to 10-5 mmHg.
And it’s one of those pieces of kit that we still use on and off. Sure, as everyone moaned, it won’t read continously, but it’s reliable provided your system is dry and you’re not working with condensible gases – i.e. those that don’t obey Boyle’s Law. In its most familiar incarnation – The Vacustat – that was introduced, I believe by Edwards some time in the ’50′s (correct me if I’m wrong) you can still find them scattered round research labs. I failed to get a photo in time for the actual column but here is one I found connected to a vacuum system. You rotate it around a pivot on the back and mercury pours from the reservoir trapping the gas in the capillary.
The peculiar thing about Herbert McLeod is that it’s remarkably hard to find out what he did. He published a bit. But not much. He also spent a lot of time helping the politician Lord Salisbury (who would later become Prime Minister) to set up a lab at his country home, Hatfield House. Together they seem to have done lots of experiments in electricity and magnetism, electrochemistry, vacuum, and whatever caught their fancy.
Anyway, you can read his story over at the RSC’s latest Classic Kit page.