The virtue of gold as an electrical contact material


The Virtue of Gold as an Electrical
Contact Material
AN EARLY DISCOVERY BY BENJAMIN FRANKLIN

Bernard Caccia

The outstanding properties of gold as a light-duty
electrical contact material are so well appreciated,
particularly in the electronics industry, that its use
is accepted without question in circuits of many
different types. But there had to be a first time for its
advantages to be discovered and understood. This
came about as long ago as 1747, in the colonial days
of North America, when that truly remarkable man —
printer, publisher, scientist, philosopher, and states-
man — Benjamin Franklin, already at the height of his
business activity in Philadelphia, not only found time
to establish this property of gold but also grasped in a
rudimentary form the concept of contact pressure and
the importance of its effect on contact resistance.

Franklin's interest in electricity had been aroused
when on a visit in 1746 to his native Boston, where he
attended a lecture and demonstration given by a
Dr Spence from Scotland. "The experiments", he
wrote, "were imperfectly performed, as he was not
very expert; but, being on a subject quite new to me,
they equally surprised and pleased me". He later pur-
chased Spence's elementary apparatus and began
experimenting for himself. His only source of
electricity was a glass sphere or cylinder rotated by a
crank and rubbing against a pad of leather or cloth,
the charge so generated being drawn to a metal bar or
chain suspended by strands of silk.

Today it is difficult indeed to realise the funda-
mental and perceptive nature of the experiments
carried out by a small band of amateur scientists in
Europe and America when their only source of
the "electric fire" was electrostatic, before the days
of continuous current from the Voltaic pile. In
October 1745 Ewald Georg von Kleist, dean of the
cathedral in the small Prussian city of Kammin
(now Kamien-Pomorski in Poland) had hit upon a
means of storing an electric charge in a small apothe-
cary's phial with a nail driven into a wooden spool
and inserted into the bottle. Almost simultaneously,

Benjamin Franklin

1706-1790

(from the portrait painted by Mason Chamberlain in 1762)

in January 1746, Pieter van Musschenbroeck, Profes-
sor of Mathematics at Leyden University, discovered
the same phenomenon, using a glass bottle filled with
water and fitted with an iron wire dipping into the
water and protruding through the cork.

This of course came to be known as the Leyden
Jar, and among those who experimented with them
in London was Peter Collinson, a Quaker cloth
merchant and a Fellow of the Royal Society with
whom Franklin had become friendly during his stay
in London from 1724 to 1726. A steady correspond-
ence had been maintained over the years between
the two, and very quickly Collinson sent over to
Philadelphia one of the new and exciting pieces of
apparatus. Franklin immediately had a number of
jars made at a local glass works, and he and his circle
of friends embarked on a series of experiments, but
it was Franklin's characteristic energy and his total
absorption in 'his investigations that carried him
swiftly to a level of understanding that equalled or
surpassed those of his distinguished contemporaries
in Europe. He explained the operation of the Leyden

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LETTERS Ott. ELECTRUCITY. t

Take a book whole cover is 5lletted with gold ;
bend n wife of eight . or ten inches long in "the form
of (m) Fis pipit on the end of thee covcrof the book over
he gold line, Co a;6 that the tlaoulder of it may prefs upon

 end of the gold line, the ring up, but leaning towards`
the other end of the boots. Lay the book oils glafs or wax
and on the other end of the gold lines, it the bottle. clec-
trifed t then bend the fpringing-wire, by prcli'ing it with a
flick of wax till its ring approaches the ring of the bottle
wire ; inftantly there is a Prong fpark and ftroke, and the
whole line of gold, which completes the communication
between the top and bottöm of the bottle, will appear a vi.
vid flame, like the (harpeft lightning. The clofer the .con-
ta (between the Ihoulder of the wire, and the gold at one.
cod' of the liite, and between the bottom of the bottle and.
the gold at the other end, the better the experiment Yuc-
cceds. The room f could be darkened. If you would
have the whole filletting round the cover appear in fire at
once, let the bottle and wire touch the gold in the diagonally
oppofite corners.

I nm, &c
B. FRANKLIN.

Franklin's letter of September 1st, 1747, to Peter Collinson describing the conducting and contact properties of gold. "Tire
passing of the electrical fire from the upper to the lower part of the bottle", lie wrote, "to restore the equilibrium is rendered
strongly visible by the following pretty experiment".

jar as a condenser, he introduced some of the terms
we still use - positive, negative, plus and minus,
conductor, and non-conductor - and of course he
showed that lightning was an electrical display. But
it is in one of his early experiments that we are
interested in connection with gold.

All Franklin's studies were communicated by letter
to Peter Collinson in London and were later pub-
lished as Experiments and Observations on Electricity
made at Philadelphia in America by Mr Benjamin
Franklin, and communicated in several Letters to Mr P.
Collinson, of London, F.R.S. It is on September Ist,
1747, in Letter 1, Experiment XI, reproduced here,

that Franklin describes his "pretty experiment" with
a book having gold filleting on the binding, coming
to the conclusion that:

'The closer the contact between the shoulder of the

wire, and the gold at one end of the line, and between

the bottom of the bottle and the gold at the other end,
the better the experiment will succeed.'

Nothing further is recorded on this initial observation
of the contact properties of gold and in fact even the
earliest and most elementary studies of electrical
contact phenomena did not begin until well over a
hundred years later.

AND I have further try'd, that with a small quantity of a certain Saline SubstanceI prepar'd, I can easily enough sublime Gold into the form of red Chrystalls
of a considerable length; and many other wayes may Gold be disguis'd, and help

to constitute Bodies of very different Natures both from It and from one another,

and nevertheless be afterwards reduc'd to the self-same Numerical, Yellow, Fixt,

Ponderous and Malleable Gold it was before its commixture.

ROBERT BOYLE

London r66r
	

The Sceptical Chymist

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