Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: a physics textbook error, a late reaction



Willem,
Thanks for tracing the history of Ohm's Law. I can push back the
start of studying resistance to about 1770, when Henry Cavendish measured
the relative resistances of different salt solutions to conducting
electricity. He did it with two Leiden charged from the same source and
discharged through two solutions to himself. He adjusted the length of the
solutions the electricity went through and when the shock felt the same he
knew the resistances were the same. His results by this method were within
5% of the modern value. Following another thread on this list - he is said
to have taken shocks with no visible reaction.
Maxwell rediscovered this work in Cavendish's notebooks, and was
startled that Henry's solutions were diluted to have the same molarity in a
system where hydrogen was one unit. Remember that hydrogen gas has a
molecular weight of two. The answer turned out that Cavendish used marble
chips (calcium carbonate) as one hundred to compare the reactivities of
various substances, and its molecular weight just happens to be one hundred.
I was also found very interesting the reference to Pouillet. He
built a magnet following Joseph Henry's design as did Faraday and a great
many others. In displaying the properties of the magnet to his students, he
used his bare hands to break the electric current. He was almost thrown off
his feet by the violent shock. He published a warning to others, but failed
to ponder the source of the shock. He thus missed being the first to
discover self-inductance, before Henry and Faraday. See, Nouveau Bulletin
des Sciences, 1832 (page 127).

Gary



Gary Karshner

St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU