Measuring the Universe

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After the invention of the Leyden jar in 1745 which allowed electric charge to be stored, electrical demonstrations became very popular. Electricity was generated using electrical machines which consisted of a glass globe or plate rubbed with a leather pad. The electricity was passed to the ‘prime conductor’, often a brass cylinder, from where it was led off to perform experiments. Demonstrations such as making hair stand on end were performed on children or servants. The electric charge was also used to produce sparks or make chimes turn by the electric wind. Electric shocks were thought to have medical benefits.

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While electricity could be entertaining and possibly beneficial, it was also potentially very dangerous. Lightning was proved to be electrical by Benjamin Franklin’s famous kite experiment in 1752. The following year a similar experiment killed G.W. Richmann, a professor at St Petersburg. Franklin’s work stimulated research into atmospheric electricity in general, and lightning conductors in particular. These researchers, together with those working on electrical machines, called for electrical measurements. Joseph Priestley wrote in 1770: 'A good electrometer is one of the greatest desiderata among practical electricians'.

John Canton made one of the first portable electrometers in 1753. It consisted of pith balls hung on linen threads kept in a box which acted as a handle. It was intended to measure the electric charge in the air near to a charged conductor. However, it could only give an indication. When William Henley described his quadrant electrometer in 1772 it was a considerable improvement on Canton’s.

I find by experience that the (Henley) electrometer answers all the purposes I have mentioned, with the greatest ease and exactness. I am now sure of the force of an explosion before a discharge of a jar or battery, which I had no better method of guessing at before, than by presenting to them a pair of Mr. Canton’s balls and observing their divergence at a given distance

Joseph Priestley

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When the quadrant electrometer was attached to an electrical machine and charged, a single cork ball hanging on a thread was repelled from the stem. A reading was given on the quadrant scale, which was usually made of ivory or boxwood. In 1787 George Adams, a leading London instrument maker, described the quadrant electrometer as 'The most useful instrument of the kind yet discovered'.

An electrometer which measured the output of an electrical machine by means of an adjustable spark gap was proposed by Thomas Lane in 1767. The electrometer was adjusted using a micrometer screw to a particular distance. When the Leyden jar was sufficiently charged from the prime conductor, the spark would jump across the electrometer. This arrangement was often used in medical applications to give patients a shock of a particular strength. The discharge was measured in inches and the capacity of the Leyden jar in gallons or pints.

In the last years of the century interest turned from measuring large quantities of charge to very small ones. Charles Auguste de Coulomb invented his torsion balance, which he used to determine the inverse square law of electrostatic force. These sensitive devices opened up new areas of research and led to much needed theoretical understanding.