1821

Electric conductivity

• Humphry Davy experimented with powerful direct currents
• from an adjustable battery of voltaic cells at the Royal Society
• consisting of two thousand double plates of zinc and copper
• in various mixtures of water and nitrous and sulfuric acids.
• In a series of experiments to determine the relations
• of different conductors to the magnetism produced by electricity,
• Humphry Davy found that induced magnetism
• was not affected by the electrification of the magnetized metal,
• or by its agitation in the case of liquid mercury in a glass tube,
• or by whatever was used to conduct the electricity.
• Furthermore, Davy realized that there was a limit
• to the quantity of electricity that a wire could transmit.
• When exceeded, the wire would turn red hot or fuse
• and the flow of electricity would stop or severely diminish.
• Davy conducted experiments to determine
• how the limit varied by the temperature, mass, surface, length,
• and induced magnetism of the wire.
• Davy found that conductivity of metallic bodies
• “was lower in some inverse ratio
• as the temperature was higher,”
• whether the heat was caused by the electricity
• or by an external flame.
• Using water to cool wires of different metals,
• Davy found that silver was the best conductor,
• followed by copper, lead, gold, zinc, tin,
• platinum, palladium, and iron,
• that the longer the wire, the less it would conduct,
• that the more massive the wire, the more it would conduct,
• and that the shape of the wire didn’t matter.
• *
• Davy wrote, “But till the causes of heat and of electricity are known,
• and of that peculiar constitution of matter
• which exites the one, and transmits or propagates the other,
• our reasoning on this subject must be inconclusive.”
• However, Davy realized that “if heat be supposed a substance,
• it cannot be imagined to be expelled from the platinum;
• because an unlimited quantity may be generated from the same platinum,
• i.e. as long as the electricity is exited . . or renewed.”

The electrical researches of the Honourable Henry Cavendish

• James Clerk Maxwell, Cavendish Professor of Physics
• at Cambridge, with access to the papers of Henry Cavendish,
• published in 1879 a complete edition and analysis
• of the private researches of Henry Cavendish.
• Between 1771 and 1781, Cavendish wrote
• of the capacity of a conductor, of the degree of electrification,
• and of the conductivity of various substances,
• “that iron wire conducts about 400 million times”
• and “sea water . . . conducts 100 times”
• “better than rain or distilled water.”
• Forty years before the invention of the galvanometer,
• which today we use to compare electrical resistance,
• Cavendish used his own body to compare the intensity
• of currents conducted through a variety of substances.
• He compared the conductivity of saltwater
• with various concentrations of salt at various temperatures
• and found for lower concentrations that heat improved conductivity
• but for higher concentrations heat made no difference.
• His studies preceded the work of Charles-Augustin Coulomb
• and were conducted with static electricity from Leyden jars.

Electric fluid

• Henry Cavendish considered electricity
• to be a liquid or elastic form of matter
• whose particles repel each other
• and are attracted to particles of ordinary matter
• (which also repel each other).
• Electric fluid is interspersed among all particles
• of ordinary matter, and normally it exists
• in a degree of saturation that balances
• attractive and repulsive forces.
• When a body has too little electric fluid,
• it is undercharged, and when it has too much,
• it is overcharged, so that the electric fluid
• can move from overcharged to undercharged bodies.
• When a body can conduct electricity,
• Cavendish considered its electric fluid to be moveable.
• And when a body can not conduct electricity,
• Cavendish considered its electric fluid to be immoveable.