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The Dry-Pile (also known as the Duluc pile or Zamboni pile) is a high voltage low current semi-permanent electric battery developed in the early 1800s and constructed from silver foil, zinc foil, and paper. Foil disks of about 2cm dia. are stacked up several thousand thick and then either compressed in a glass tube with endcaps and a screw assembly, or simply stacked between three glass rods with wooden endplates. This is a simple type of Voltaic pile, a multi-cell electrochemical battery, with an output potential in the kilovolt range. In effect it is an electrostatic battery.
The dry pile grew out of the debate between those that ascribed the electrical behaviour of the wet voltaic pile either to contact tension or a new type of electrochemical action.
A number of high voltage dry piles were invented between the early 1800s and the 1830s in an attempt to determine the source of electricity of the wet voltaic pile, and specifically to support Volta’s hypothesis of contact tension. Indeed Volta himself experimented with a pile whose cardboard discs had dried out, probably accidentally. The first to publish was Johann Wilhelm Ritter in 1802, (albeit in an obscure journal) however over the next decade it was announced again and again as a new discovery.
Ritter decided to follow up his observation that the voltaic pile continued to exhibit some electrical potential even after its moist conductor had almost completely dried out. He constructed a dry pile made up of 600 pieces of zinc, copper and white sheep’s leather which appeared to be free of moisture. This pile charged a Leyden jar to the same degree as a wet pile of the same size, and the spark and shocks produced by this jar were of the same size. The main differences were that such a pile took much longer to charge the jar. After further investigation he concluded that it was the moisture of the cardboard, leather or any other intermediate substance that made the pile electrically active, and that only the smallest degree of moisture was required.
Others tried to construct a real dry pile in order to refute Ritter’s claim that a voltaic pile had to have some moisture for it to be electrically active but it was found extremely difficult to exclude all moisture from the experiment. In particular it was noted that the pile's performance was influenced by the weather. Paul Erman in a detailed paper published in 1807 demonstrated that this electrical instability came from variations in the moisture content of the so-called dry cardboard caused by changes humidity. Thus, the dry pile could be regarded as a special kind of hygrometer in which electrical activity was related to moisture. Attempts to turn this effect into a practical instrument failed owing to the lack of an accurate method to measure voltage.
By 1807 the main properties of the dry pile had been established. It was noted that both the voltaic and the dry pile produced electricity, but there were differences in the electrical effects. In the case of the dry pile the phenomena were more akin to the high voltage electricity produced by frictional electrostatic generators, but it was electricity nevertheless.
In the next wave of experimental activity, in particular by Jean-André Deluc and Giuseppe Zamboni, (despite the fact that history has affixed their names to the device), little more of value about the general behaviour of this device would be discovered, although probably the most efficient dry piles were constructed by Zamboni. Zamboni was particularly keen to devise a pile that could move a light pendulum for a very long time. The pendulum was mounted between the oppositely charged poles of two piles placed side by side, and went into oscillation because of the alternating attraction and repulsion it experienced. The device became the motive power of an electrostatic clock utilising the oscillating bob between the poles. but these never went beyond being scientific curiosities. A few incomplete Zamboni clocks have survived. The dry piles were also used in a small group of highly sensitive single gold leaf electroscopes that could also indicate polarity.
Notable other uses
The Clarendon Dry Pile set up in 1840 at Oxford University in England has been ringing a bell twice a second from then to the present. It has been estimated that the bell has been struck somewhere in the order of 10 billion times. 
Dry piles found commercial use as the power supplies of electrostatic voltmeters (quadrant electrometers), and in infrared converter night vision goggles used in World War II.
- Willem Hackmann, "The Enigma of Volta's "Contact Tension" and the Development of the "Dry Pile"", appearing in Nuova Voltiana: Studies on Volta and His Times Volume 3 (Fabio Bevilacqua; Lucio Frenonese (Editors)), (2000) pp. 103-119
Categories: History of physics | Electric batteries