bottom of the third column on the left with a capillary tube. The centre column is initially completely filled with mercury, as is the connecting capillary. The other two columns are partially filled. The top of both the main column on the right and the reservoir column on the left are connected together and to an inlet for the pressure to be measured. When the test pressure is applied, the mercury rises in both the left and right columns and falls in the centre column. The mercury at the top of the capillary breaks and a vacuum forms there. The pressure is then measured in the usual way by the difference between the heights of the right and centre columns. Since a new vacuum is formed each time a measurement is made, there is no problem with the vacuum becoming contaminated. Any bubbles that do form in the capillary are easily removed by inverting the gauge and shaking or tapping. High pressure measurement For extremely high pressures, the column can still be very high, even when using mercury. Gauges for measuring pressure in the range have been built. A 23-metre-tall mercury column is difficult to read and suffers from inaccuracies caused by different parts of the column being at different temperatures. A more compact mercury pressure gauge suitable for high pressure was built by Heike Kamerlingh Onnes, the discoverer of superconductivity. This consisted of a series of mercury filled U-tubes connected together with inverted U-tubes. The inverted U-tubes contain compressed air at a pressure designed to bring the instrument into the pressure range of interest. The pressure is found from this instrument by summing together the difference in column heights in each of the U-tubes. History The parent of all mercury pressure gauges is the mercury barometer invented by Evangelista Torricelli in 1643. An early engineering application of the mercury pressure gauge was to measure pressure in steam boilers during the age of steam. The first use on steam engines was by James Watt while developing the Watt steam engine between 1763 and 1775. This engine was a development of the popular Newcomen atmospheric engine. A gauge for use on steam engines very similar to the later Kamerlingh-Onnes gauge was patented in 1858 by Thomas Purssglove. Like the Kamerlingh-Onnes device, it had multiple U-tubes connected in series. The connecting tubes were filled with an incompressible fluid. The instrument was formerly