Winchester College Publication Treausry: Collections Bulletin 2019-2020 | Page 9

OBJECTS IN FOCUS
Early Experiments with Radio Waves
Hertz Radio Apparatus
The German physicist Heinrich Hertz (1857–94) first demonstrated the
existence of radio waves in 1887. This replica of his apparatus dates from
early in the 20 th century and was probably built by Physics technician Walter
Abley at the instruction of William Bleadon Croft (known as ‘The Bleeder’),
Winchester’s first ever Physics teacher. Croft’s notebooks contain many entries
and journal clippings relating to Hertz’s experiments and he corresponded with
and contributed to learned societies of the day, especially the Physical Society
of London.
Hertz radio apparatus, probably made by Walter Abley, early 20th century (A0399, A0400)
An induction coil generates a high-voltage
spark between the terminals of the
transmitting plates, causing a radio wave to
travel across the lab. The magnetic field
associated with this wave induces a voltage
between the two ends of the metal receiver
loop. This can produce a small spark; in this
case a small bulb has been connected across
the gap and will flicker when it detects a
signal. So a Morse message can be
transmitted and detected—wirelessly!
In recent years we’ve regularly demonstrated
this apparatus, particularly in Div lessons
(Winchester’s multi-disciplinary teaching
programme) relating to the development of
19 th - and 20 th -century science and
technology. Whereas the wire telegraph was
the premier communication technology of
the 19 th century, allowing instant point-topoint
communication and the central
administration of large empires, wireless was
the ‘killer app’ of the 20 th century, allowing
messages to be broadcast directly to a mass
population for good or ill.
Branly Coherers
These glass tubes filled with metal filings are
known as ‘Branly coherers’ after their
French inventor, Édouard Eugène Désiré
Branly (1844–1940). They were an early
device for detecting radio waves, which
W. B. Croft (1851–1928),
Winchester’s first Physics
teacher
Pair of Branly coherers, made by
William Bleadon Croft or Walter
Abley, 1892 (A0149)
Hertz had first intentionally produced in
1887.
The tube is connected to a small battery, but
very little current flows since the filings are
separate from each other. Next, a spark is
produced (in this case by an induction coil),
generating electromagnetic waves. As these
pass through the tube their electrical field
causes the metal filings to clump together or
‘cohere’, which greatly reduces their
resistance. The increased current is visible on
the galvanometer and will persist until the
filings are ‘decohered’ by tapping them. With
a bit of ingenuity this can be used to send
wireless messages in Morse code; indeed,
Guglielmo Marconi used similar coherers in
his early detectors.
The aforementioned Physics teacher, W. B.
Croft, corresponded with several leading
scientists of the day, including the British
physicist Sir Oliver Lodge (1851–1955), and
was directly involved with these
developments. He was the first person in
England to demonstrate this effect, using
these very coherers, at the Physical Society
of London in 1893.
The coherers featured in an episode of BBC
Four’s ‘Victorian Sensations’ that aired in
June last year, in which the presenter
Philippa Perry used them to demonstrate
one of Lodge’s pioneering experiments with
electromagnetic waves.
Jeremy Douglas,
Head of Physics
Winchester College Collections 2019 – 20 9