by Andrea Sella
It has long been said that nature abhors a vacuum. Following hard on the heels of the heretical astronomical observations of Galileo, the Papal authorities were so rattled by the implications of Torricelli’s early 17th century experiments with a column of mercury in a sealed tube - could the empty space imply a denial of the Divine Presence? - that they banned them, and much other science, altogether. It was left to Protestants in northern Europe to make progress in this near-heretical field.
Around 1650, Otto von Guericke used what most people would recognise as a reversed bicycle pump to suck the air out of a metal sphere. It promptly collapsed, to Guericke’s great delight. He went on to stage the famous experiment in which opposing teams of horses were unable to separate two evacuated hemispheres. In England, Robert Boyle and Robert Hooke designed an improved pump and began the first ’real’ experiments in vacuum. Such experiments with vacuum, often involving small animals, became fashionable conversation pieces. One is famously portrayed in Joseph Wright’s 1768 painting, An Experiment on a Bird in the Air Pump. Yet in spite of steady improvements in design it seemed impossible to go lower than 1/4 Torr. The challenge was to beat that.
The first serious progress was made by Julius Plücker, a pioneer of the study of electric discharges through gases. Plücker instructed Johann Geissler, the famous glassblower, to build him a pump. The idea was simple - by raising a reservoir, a mercury column could displace part of the gaseous contents of a gas system upwards past a non-return valve. By alternately raising and lowering the reservoir the system could be steadily evacuated. But while fairly effective, the Geissler pump was incredibly slow and tedious.
In 1865, the Hanover-born chemist Hermann Sprengel (1834-1906) working in London, presented a stunningly simple apparatus which could operate with little or no supervision and could achieve lower vacuum than anything else available at the time. The device (right) was a stroke of genius. The system to be evacuated was connected to a T-piece. Mercury was made to flow from a reservoir, past the T-junction, and down a capillary in the form of a thin stream of droplets. Each droplet entering the capillary entrained a small amount of air from the vacuum system and lowered gas pressure steadily until the approach to a ’perfect’ vacuum was signalled by steady knocking sounds, and in a darkened room, ghostly flashes from within the apparatus.
The Sprengel pump was so fast and so efficient - Sprengel’s earliest attempts could completely evacuate a half litre vessel in 20 minutes - that everyone working with vacuum used them. William Crookes used the pumps in series in his studies of electric discharges. William Ramsay used them to isolate the noble gases. But, most significantly, Swan and Edison evacuated their new carbon filament lamps with Sprengel pumps. Sprengel himself moved on to investigating explosives and was eventually elected as a Fellow of the Royal Society.
But what was the pressure? Sprengel knew that it was so low that it no longer supported an electric discharge, but beyond guesstimating a millionth of atmospheric pressure (10-3 Torr), he was stuck.
The second breakthrough - measuring vacuum - was achieved by London-born Herbert McLeod (1841-1923). In 1874 he developed a simple gauge based on the Geissler pump: a raised mercury column would trap a small sample of gas from the evacuated system and compress it into a capillary, where its volume would be measured; working backwards, one could calculate the pressure in the original vacuum.
It soon became clear that pressures of the order of 10-6 Torr were being reached. But the gauge’s design was attacked repeatedly. Morris Travers, who used it with Ramsay, complained that it was very sensitive to contamination by moisture, and needed recalibrating for each gas. Others objected to it not providing continuous readings.
By the start of the 20th century electrical mercury and oil-sealed rotary pumps had begun to appear and Sprengel’s pump, along with its inventor, gradually faded from memory. But the McLeod gauge, in spite of its shortcomings, is still the method of choice for calibrating modern electronic vacuum gauges. And what of the Church’s antipathy to vacuum? As incandescent lights came on all across Europe the Pope unobtrusively bowed out of the discussion.
Andrea Sella is a lecturer in inorganic chemistry at University College London