- Museum of the History of Science - http://www.mhs.ox.ac.uk -

A Torsion Balance Electrometer by Watkins & Hill

Autumn, 1997

IN the basement of the Museum, in a case with other electrical instruments, is a small torsion balance electrometer, made in 1845 by Watkins & Hill, instrument makers in London.

The torsion balance electrometer was developed by Charles Augustin Coulomb (1736-1806) in a series of experiments carried out in Paris in the years before and during the Revolution and culminating in his prize-winning memoir purporting to demonstrate the inverse square law of electrostatic force.

The primary use of the balance was in measuring very small charges and estimating the attractive and repulsive forces between bodies of known surface area. It generally consisted of a horizontal insulating needle (missing in the Museum’s instrument) with a small ball of conducting material at one end and counterpoise at the other, suspended in a glass receiver at the end of a thin thread. Bodies were introduced into the receiver next to the ball and their charge measured by the degree of deflection of the indicator needle or, to be more precise, by the torsion under which the thread must be placed in order to bring the needle back to its original position.

Although promoted as an instrument that ‘yields accurate estimates of the forces between charged bodies’, the advertised accuracy of the Watkins & Hill balance is somewhat questionable, especially when one considers that at 14 inches in height and 5 inches in diameter it is nearly three times smaller than the balance used by Coulomb, which was 12 inches in diameter and 36 inches in height.

Watkins & Hill, in reference to the balance in their catalogue of instruments, make a passing remark about ‘the requisite precautions’ that must be taken in measuring electric force with the instrument. However, exactly what these precautions are is not stated, being left, presumably, to the judgement of the experimenter, based on his skill and practical knowledge.

The difficulty in using the torsion balance and the problematic nature of the replication of Coulomb’s experiment was illustrated in 1992 in a programme carried out by Peter Heering at the Department of Physics in the University of Oldenburg. Heering built a torsion balance as similar as possible to the one used by Coulomb and tried to replicate Coulomb’s experiment according to the description given in his celebrated 1785 memoir.

In attempting to replicate the experiment, Heering encountered several unexpected problems. Apart from the obvious difficulty of making a balance identical to Coulomb’s (due to the unavailability of original materials), the experimental procedure itself appeared much more problematic than was apparent from Coulomb’s memoir.

For example, as soon as the ball in the balance is charged by an external conductor, the balance moves because of the mechanical impulse of the contact. External vibrations may also cause the balance to move, disturbing the measurements. The body of the experimenter is another unavoidable source of disturbance, this time electrical. Moreover, if the charge communicated is too strong, the thread in the balance is very likely to break.

After completing his trials, Heering concluded that it was impossible to use the balance to obtain results that fit the inverse square law of electrostatic force without using a Faraday cage – a piece of equipment invented half a century after Coulomb had completed his work – to shield the balance from unwanted electrical interference. Coulomb’s supposed demonstration of the same law on the basis of just three measurements taken with the torsion balance appears all the more intriguing.

Heering uses the difficulties encountered in the replication of Coulomb’s experiment as a tool to analyse the general scepticism and reluctance of early nineteenth-century German physicists towards the inverse square law. The reactions of other European experimenters are the subject of contributions to Restaging Coulomb, a volume edited by Christine Blondel and Matthias Dörries and published in Florence in 1994. The focus of this work is a critical review of Heering’s replication, but general questions on the relationship between the replication difficulties encountered today and those faced by past experimenters are raised throughout the volume.

A common line of several contributions to the volume is that the replication of historical experiments gives insight into the ‘tacit knowledge’ underlying them, knowledge which cannot be inferred either from Coulomb’s report or from that of any other experimenter of the late eighteenth-century.

Controversial as it might have been, Coulomb’s balance became the symbol of the inverse square law of electrostatic force and was ever-present in nineteenth-century teaching laboratories. It was a success commercially, with instrument makers including it in their catalogues throughout the cen tury. The price of Watkins & Hill’s balance was 2 pounds 2 shillings, which made it one of the most expensive electrostatic devices available at the time – about the same price as a 6- inch plate electrical machine.

The adjective ‘accurate’ was inextricably linked with the Coulomb balance. Besides Watkins & Hill, other instrument makers in London exploited the rhetoric of accuracy and precision associated with it. E.M. Clarke described it in his catalogue of 1850 as an ‘accurate instrument for measuring small quantities of electricity’ and in 1878 Negretti & Zambra advertised it as an instrument that, with ‘careful manipulation’, will give ‘accurate values of the attractive and repulsive force of free electricity communicated to any body of known area’.

In none of the London instrument makers’ catalogues is the size of a torsion balance ever mentioned, strengthening the impression that its proclaimed accuracy was a rhetorical expedient exploited by its makers and shared by nineteenth-century scientists, regardless of its actual use. How often it was actually used for the replication of Coulomb’s experiment is difficult to determine – in nineteenth-century French works on electricity it is Coulomb’s own words rather than contemporary reports that are used to describe the experiment. Similarly, in English textbooks it is quite common to find a translation of Coulomb’s original paper.

Despite opposition to Coulomb’s results from other experimenters – Volta in Italy and Deluc in England for ex ample, who belonged to a different experimental tradition and doubted their value – the torsion balance had a wider significance for natural philosophy through the technique of measuring forces by the angle of torsion of a thread. The ‘torsion technique’ was used in nineteenth-century laboratories in other kinds of electrical apparatus, such as galvanometers and Thomson’s quadrant electrometer. Its sensitivity was exploited in the measurement of magnetic and gravitational forces as well as in electrical precision measurement.

At the time of the publication of his memoir, Coulomb had already established his authority as a skilled experimenter, thanks to his use of a torsion balance in magnetic measurements. The prevalence of the balance as an ornament in nineteenth-century laboratories was certainly due in part to its association with Coulomb as an authority, but its popularity must also have rested to a significant extent on the promise for future development that it came to represent.

More than anything else, the inclusion of the torsion balance in the range of products sold by instrument makers two thirds of a century after its invention, and its commercial success, point to the role that instruments can play in encoding symbols and values of laboratory life.

Paola Bertucci