Preliminaries:

I expect this to be a first in a series of experiments dealing with electrometric radiation. I would place it immediately after a consideration of induction and before optics.

Prelab Questions:

1)         Explain the difference between induction and electromagnetic radiation.

2)         How would you be able to generate high frequency electromagnetic oscillations (greater than 1,000 Hz)?  (Leading to Fessenden's mechanical generator, Hertz's spark gap, and DeForest's vacuum tube.)

3)         Why do you suppose spark-gap radio transmitters are outlawed by the FCC?

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Physics ### lab

Electromagnetic Propagation - Hertzian waves

Purpose: The purpose of this experiment is to investigate Hertz' original experiment, whereby he reported to have detected radio waves.

Equipment: For this experiment, you will need:

*          High voltage generator, (coil)

*          Dipole radiator with spark gap.

*          Loop receiving antenna with spark gap.

Background:

In 1858 Wilhelm Feddersoen (1832-1918) German physics professor at Leipzig demonstrated experimentally that the sparks jumping between the spheres of a Ruhmkorff coil oscillated and that they could generate electric waves of long wavelength. In 1861, he used the rotating mirror designed by British physicist Charles Wheatstone (1802-1875) to record the phenomenon on a photographic plate.

Using mathematical models, James Clerk Maxwell had suggested that two different types of electrical disturbances could possibly exist in Nature. One type was a longitudinal electric wave which required alternating concentrations of densified and rarefied pulsations of electrostatic fields that moved along a single vector (today, we refer to these as standing waves or scalar waves). Maxwell ultimately rejected this idea because he was convinced that this type of wave propagation was impossible to achieve, but his assumption was erroneous.

Maxwell’s second wave postulation was that of a transverse electromagnetic wave that exhibited a rapid alternation of electric fields along a fixed axis that radiated away from its point of origin at the speed of light and was detectable at great distances. Maxwell had more faith in the existence of this type of wave and encouraged experimenters to look in this direction. It was the discovery of this type of wave that Hertz laid claim to.

In 1888 Hertz drew an influential series of diagrams to accompany his 1889 paper on dipole radiation, which was translated as "The forces of electric oscillations, treated according to Maxwell’s theory". Hertz took a great deal of care with these drawings, (below).

You can see that the focus of Hertz's discussion is on the radiated EM field. He spends little time discussing the actual dipole antenna or the fields immediately close to its surface.

Below is a schematic diagram of Hertz's transmitter. You can see that he used a Ruhmkorff coil to induce a huge voltage across the spark-gap in the antenna.

You see, below, the setup for detecting Hertzian waves.

The graph shown in the figure below shows the relationship between the magnetic (H) field and the electric (E) field plotted against time. Note that the two fields are 90 degrees out of phase with each other in time. It is also true that the two fields around the antenna are displaced 90 degrees from each other in space. The H field exists in a plane perpendicular to the antenna. Its field lines circle the axis of the antenna. The E field exists in a plane parallel with the antenna. Its field lines arc along the length of the antenna.

Phase Relationship of Induction Field Components

All the energy supplied to the induction field is returned to the antenna by the collapsing E and H fields. No energy from the induction field is radiated from the antenna. Therefore, the induction field is considered a local field and plays no part in the transmission of electromagnetic energy. The induction field represents only the stored energy in the antenna and is responsible only for the resonant effects that the antenna reflects to the generator.

The diagram below is a simple picture of an E field detaching itself from an antenna. (The H field will not be considered, although it is present.) In view A the voltage is maximum and the electric field has maximum intensity. The lines of force begin at the end of the antenna that is positively charged and extend to the end of the antenna that is negatively charged. Note that the outer E lines are stretched away from the inner lines. This is because of the repelling force that takes place between lines of force in the same direction. As the voltage drops (view B), the separated charges come together, and the ends of the lines move toward the center of the antenna. But, since lines of force in the same direction repel each other, the centers of the lines are still being held out.

Radiation from an Antenna

As the voltage approaches zero (view B), some of the lines collapse back into the antenna. At the same time, the ends of other lines begin to come together to form a complete loop. Notice the direction of these lines of force next to the antenna in view C. At this point the voltage on the antenna is zero. As the charge starts to build up in the opposite direction (view D), electric lines of force again begin at the positive end of the antenna and stretch to the negative end of the antenna. These lines of force, being in the same direction as the sides of the closed loops next to the antenna, repel the closed loops and force them out into space at the speed of light. As these loops travel through space, they generate a magnetic field in phase with them.

Since each successive E field is generated with a polarity that is opposite the preceding E field (that is, the lines of force are opposite), an oscillating electric field is produced along the path of travel. When an electric field oscillates, a magnetic field having an intensity that varies directly with that of the E field is produced. The variations in magnetic field intensity, in turn, produce another E field. Thus, the two varying fields sustain each other, resulting in electromagnetic wave propagation.

During this radiation process, the E and H fields are in phase in time but physically displaced 90 degrees in space. Each field supports the other, and neither can be propagated by itself. The figure below shows a comparison between the induction field and the radiation field.

E and H Components of Induction and Radiation Fields

Procedure:

Work in teams of at least two people.

1)         Assemble the apparatus as described above.

2)         Close the switch to the coil in order to produce a high voltage spark between the legs of the dipole radiator.

3)         View the minute resultant spark (in a dark room) in the gap of the receiving antenna.

4)         Turn the receiving antenna 90° and try the experiment again. Record your `        observations.

5)         Adjust the gap spacing and record your observations.

6)         Adjust the voltage supplied to the coil (and thus the power of supplied to the spark) and record your observations.

Follow up Questions for Students:

1)         How does the size of the antenna relate to the frequency of radio signal emitted?

2)         Does the antenna radiate an electrical or magnetic field?

3)         Would it be possible to design an antenna for either electric or magnetic radiation? How would they differ in design?

4)        Why did the receiving antenna look different than the radiating antenna?

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Students generally assume that radio wave transmitters need to be complicated systems of electrical components networked in some unfathomable arrangement. They should be surprised at the simplicity of this experiment.

I tried to copy Hertz's setup as closely as possible. I had trouble finding details of his apparatus. The Web pages I found with more than token information were written in German or higher math, (both languages I have difficulty with.)

Above, you see the dipole radiator, Hertz's big invention. I tried to match his in size and appearance. I was surprised that they still sell copper float bulbs at the hardware store.

I used a Wimshurst Generator to supply the several kilovolt spark to the gap between the antenna legs. Nice spark, but not nearly enough power for the job.

Hertz used a Ruhmkorff coil. Wilhelm Feddersoen had already shown in 1858 that the sparks jumping between the spheres of a Ruhmkorff coil oscillated. Hertz figured that they could generate electric waves.

I positioned the receiving antenna away from the "near field". We wouldn't want to confuse induction with radiation.

This is a close-up of the gap in the receiving antenna. I used ball bearings. They don't look shiny anymore because I had to heat them up (anneal) before I could drill them. You can see the big capacitor on the table. I tried using that to store more energy. It didn't help. I couldn't get it to charge at all with that generator. I should have brought in a disposable camera.

Here I am, working real hard. The sparks look cool. But even with the lights off, we couldn't see any spark in the receiver. I wasn't surprised. I would like to try a Ruhmkorff coil some day. Here's a picture (below) of a big one that maybe I can find on e-bay.

I did find an air-core inductor in the physics lab. I've made sparks with coils before and thought this was worth a shot. I fed DC to it. Then pulled the power. I hoped that the collapsing field would induce enough emf to jump powerfully. It didn't work.

So, my results were not good. I assume that I was not using enough power. I will try again with a large coil and higher current.

Another idea is to use a camera with the shutter open to record one or more sparks.

Sources

Unraveling Tesla’s Greatest Secret

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Reflections on Hertz and the Hertzian Dipole

Jed Z. Buchwald, MIT and the Dibner Institute for the History of Science and Technology

http://dibinst.mit.edu/DIBNER/DIConferences/OldConferences/Sloan/reflecti.htm

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HISTORICAL EXPERIMENTATION

Taught in 1999 by Jed Buchwald at the Massachusetts Institute of Technology

Which states:

o. Hertz propagation experiments as originally performed. Easy to set up once resonator and oscillator in hand.

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## History

http://dspt.club.fr/antiqueR.htm

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Heinrich Rudolf HERTZ

http://dspt.club.fr/hertz.htm

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K14 Electric oscillations

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http://www.tpub.com/neets/book10/40.htm

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Heinrich Daniel Ruhmkorff

http://chem.ch.huji.ac.il/~eugeniik/history/ruhmkorff.htm