Early Experiments with Radio Waves
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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