well as the mathematical and theoretical
knowledge to understand and interpret
the findings.
Influenced by many theories and
experiments on electromagnetism, Hertz
first faced the problem of how to generate very rapid oscillating electric fields.
In 1857, Berend Wilhelm Feddersen
working in Kiel showed that a spark
discharge from a Leyden jar (a type of
condenser) is really a train of damped,
rapid oscillations.
Hertz attributed his initial observation with a pair of Riess spirals as
the start of his series of experimental
designs that resulted in his discovery
of propagating electromagnetic waves.
In preparation for his lectures at Karlsruhe, he located a pair of planar copper
windings in the form of a tight spiral. A
Riess spiral is made from a pair of copper
insulated wires that are wound in a tight
spiral, and each wire is terminated in a
metal sphere.
When he used a low-voltage battery
and discharged the current across the
leads of one spiral, he observed sparks in
the second, noncontacting Riess spiral.
That was the start of an intense experimental research program that Hertz
Hertz’s motivation in
conducting experiments
on electromagnetism was
to devise an experiment
that could determine if
free space behaves like
other dielectrics.
pursued for the next three years. He then
unwound the two copper wires of the
spiral to make a linear device from the
pair of wires.
After making a series of instrument
modifications, Hertz ended up with a
primary copper wire rectangle that had
a spark gap to discharge a source of
high voltage. In this way, he produced
electrical oscillations in the primary wire
rectangle. The detector was a similar
rectangle of copper wire with a small
spark gap in which the gap was carefully
controlled by a micrometer.
Finally, in 1887, Hertz was able to
detect sparks in the detector rectangle
even when there was no electrical contact
to the source copper rectangle (source).
Hertz correctly interrupted the spark
(source) as a fast-response switch between
the primary copper rectangle and the
source of high voltage. By varying the
resistance and the capacity of both parts
of the circuit, Hertz achieved resonance
between the primary copper wires with
the spark gap that was connected to the
high voltage source and the detector
copper rectangle with the micrometer
spark gap.
In 1851, Heinrich Daniel Rühmkorff
invented a device to produce extremely
high voltages; it consisted of a step-up
iron-core transformer with a an automatic switch in the primary winding
that induced a pulse of high voltage and
a spark between the ends of the secondary winding when the primary current
is interrupted.
Hertz used the eponymously named
Rühmkorff coil and its battery power
supply as a generator of sparks. He built
a lead-acid storage battery from 230
cells as a current source for the Rühmkorff coil. He then constructed a spark
micrometer, which he called a
Funken-mikrometer, so that the output gap could
be carefully measured with a precision
micrometer. In the fall of 1886, Hertz
was using this equipment for his investigations of electrical phenomena in gas
discharge tubes.
The primary circuit or the transmitter, driven by the spark discharge of the
induction coil (Rühmkorff coil) was a
copper wire in a closed loop with a small
air gap. The spark produced rapid oscillations of voltage in the wire loop; these
voltage oscillations were the source of
the radiation. Hertz found that the spark
discharge rapidly made the resistance of
the gap zero. The receiver was a similar
copper wire loop with the air gap at the
ends. Hertz obtained shorter wavelengths
of the electric field by lowering the
inductance and the capacitance of his
circuit so he could measure the standing
waves in his small laboratory.
Hertz constructed several configurations of the basic spark apparatus; the
source was the Rühmkorff coil, and this
was electrically connected to rectangular
34 | OPN Optics & Photonics News
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