Types of Barometer

Many barometer designs have been developed since the invention of the instrument. Issues of portability, scale and accuracy forced natural philosophers and instrument makers to think about how to best enhance the measuring mechanism. This section covers three types of barometer design: cistern, angle or diagonal, and aneroid.

Invented by Herman von Helmholtz, these instruments were used to analyze the composition of musical and speech sounds by means of 'resonances'. Later improved by Rudolph Koenig (1832-1901), they were an essential tool for the 19th century acoustician.

This ingenious device was the very first sound synthesizer: a tool for studying and artificially recreating musical tones and the sounds of human speech.

Background

Suppose I sing the word 'car' and then on the same note sing 'we'. The two vowel sounds will be similar in so far as they have the same pitch yet they have a clearly distinct sound quality, or timbre. What is it that accounts for this difference, and the timbres of musical sounds in general? Helmholtz set out to answer this very question in the mid 19th century, building on the work of the Dutch scientist Franz Donders (1818-1889).

This acoustic interferometer, reminiscent of a trombone, is for experiments and demonstrations relating to the interference of sound waves. From an original design by Sir John Herschel (1792-1881), this example was built by the Parisian scientific instrument maker Rudolph Koenig (1832-1901) in 1889.

Cistern barometers

In 1695 Daniel Quare secured the first patent for a portable barometer. Quare's design was sealed, to prevent spillage of mercury, and it used a cistern to provide a mechanism for adjusting the volume of the mercury in the barometer tube (Image 1).

However, The Clockmaker's Company originally opposed Quare's patent, claiming that Joachim d'Alsace had published a description of a similar design in 1688 and that the English clock-maker Thomas Tompion had already produced such as barometer. Nevertheless, Quare was granted the patent and began manufacturing the design.

His cistern barometer consisted of a sealed hollow cylinder of boxwood - a material pervious to air along the grain yet impervious to mercury. At the bottom of the cistern, a leather bag rested on a padded end of a screw. By adjusting the height of the screw, the leather bag could be made to inflate or deflate, thus affecting the mercury level in the tube. When the screw was advanced so that the entire tube was filled with mercury, the barometer could be turned upside-down and carried away.

Though Quare's cistern barometer improved portability and the capacity to ship the instrument without incurring damage, the short length of the barometer tube suggests that it was intended for use in a domestic rather than mountain-top setting.

Angle or diagonal barometers

The angle or diagonal barometer is a simple modification of the original tube barometer, which expands the barometer scale and improves reading of the instrument. The angle barometer shown in Image 2 was made by Thomas Page (d. 1784) of Norwich, a fashionable and wealthy area of England during the eighteenth century.

Angle barometers such as this were commonly found hung in the home parlour. Shaped as an inverted 'L' with a mercury cistern at the base, the tube is bent at 27 inches so that the extending arm is less than ninety-degrees. The angle of the arm tube increases the apparent movement of the mercury, since the mercury must rise through more of the tube in order to reach the same height as in a vertical tube.

Manipulations of the tubing, such as curved or elliptical glass, magnified the mercury and enabled easier reading of the instrument.

The mercury barometer and thermometer by Angelo Lovi (c. 1730-1805) shown in Image 3 uses tube curvature to render the instrument easier to read. Lovi emigrated from Milan to Edinburgh in 1772. His excellent knowledge of traditional Italian glass-blowing techniques easily transferred to the making of unusually shaped barometric tubes.

Aneroid barometers

The aneroid barometer measured air pressure according to the movement of mechanical parts, rather than the expansion of a liquid, and it is capable of produced readings more accurately than spirit or mercury barometers.

The concept of the aneroid barometer was initially suggested by Florian Périer's brother-in-law, Blaise Pascal (1623-1662). Several designs were proposed by various members of the Royal Society, but early modern instrument makers were unable to produce these delicate and sensitive instruments.

In 1844, the French lawyer and engineer Lucien Vidie (1805-1866) successfully produced a metallic barometer that used springs to measure atmospheric pressure. It consisted of a brass box enclosed by a diaphragm. In the example in image 4, a curved thermometer sits along the bottom of the dial face.

Being slightly evacuated (meaning that some of the air was removed from within it), the box contained springs that supported the diaphragm and would compress according to air pressure exerted upon it. Registered to a pointer, these tiny movements were noted on the dial face. A bimetallic strip was employed to compensate for temperature change.

Vidie had difficultly manufacturing and selling his aneroid barometer to the French market and turned to the English clock-maker Edward John Dent (1790-1853). With Dent's help, aneroid barometers became popular items amongst English sailors and mountaineers.