Volta’s electrometer and the ‘imperceptible electricity’

Dr Christine Blondel

During several years, Alessandro Volta looked for what he called a ‘real experimentum crucis’ which would exhibit the existence of an electrical tension when two different metals are put in contact. This was supposed to give the final blow to Galvani’s animal electricity, but Volta had to acknowledge that this metallic electricity was ‘obscure and imperceptible to ordinary instruments’.

I will examine Volta’s strategy when dealing first with ordinary instruments, i.e. the usual electrometers, then with the later duplicator, and finally when devising a new experimental procedure which led to a new instrument, his condenser-electrometer. However, the difficulties encountered by the French savants to use his electrometer - and Volta’s own repeated cautions - suggest that the instrument incorporated both Volta’s mastery of weak electricity and a special craftsmanship’s expertise, especially with varnish layers.

Confusions with coils: Johann Schweigger and learning from loops

Dr Elizabeth Cavicchi

Making and using an instrument of our own involves us in confusions of learning from natural phenomena that are not easy to describe without having the experience. Just to bend one wire into a loop can become confusing in practice. When an electric current passes through the loop, it gives rise to a magnetic effect that depends on how the loop is wound. In 1821, Johann S.C. Schweigger (1779-1857) explored these effects with a double-loop instrument of his own design. Its two wire loops are confined to a plane; this puzzled me until I began experimenting with my own wire loops and realized how confusing the loop’s spatial properties can be. For subsequent nineteenth-century investigators, who made electrical coils by building up more loops, layers, and separate wire lengths, the loop exhibited new, confusing three-dimensional effects. In following those accounts through winding and testing my own wire coils, I became aware of consistencies in what it is to learn with loops. I hope these insights about learning can encourage teachers and students to bend wires and experiment with making instruments.

Charles Pritchard’s wedge-photometry at Oxford Observatory

Dr Klaus Staubermann

Based on my training as an astronomer, and drawing on resources such as museums, instrument collections and archives, I am rebuilding Charles Pritchard’s astrophotometer. Pritchard designed a wedge photometer which he used in the second half of the 19th century, first at the Oxford Observatory and later in Cairo. I am practising with the replica to get a better understanding of the instrument and the employed practice. Observers from different backgrounds were asked to participate in these experiments and to share their visual experience. This work shows how observers could agree on their experience by referring to the instrument and if an accuracy could be achieved comparable to other photometers at that time. However accuracy varied when it came to observations with different photometers, e.g. Edward Pickering’s at Harvard or Gustav Müller’s in Potsdam. In my current research I investigate the different concepts of instrumental practice in these late nineteenth-century astrophotometries. I focus on the debate between various leading astrophotometrists, on their deviating measurement results and the possible reasons. All had different concepts of the working of their instruments and their scientific practice, Pritchard used a wedge photometer, Müller using a Zöllner photometer and Pickering both a wedge photometer and a modified Zöllner photometer. This comparative study will help us to understand concepts of scientific practice, how they were communicated and how they can be accessed by historians of science.