Is a Spark Necessary for the Production of EM Waves in the Hertz Experiment?

[Zaya Bell]

TL;DR Summary

About the spark in Hertz creation and detection of EM waves

Hello, so I was reading bout the Hertz experiment in different text and they always say something about the Spark causing accelerated electric charge. So I was wondering, is the spark necessarily for the production of EM waves? If yes, why? Because as I recall, applying an ac voltage should create oscillating charge in the two electrodes which should create em waves with or without a spark. Please, tell me how I'm wrong.
Thanks in advance.

 

[Dale]

Summary:: About the spark in Hertz creation and detection of EM waves

So I was wondering, is the spark necessarily for the production of EM waves?

The spark is not necessary for EM, but it is sufficient.

 

[Zaya Bell]

The spark is not necessary for EM, but it is sufficient.

So, I can create and detect em wave from a 220 V 50Hz ac in my house?

 

[Dale]

So, I can create and detect em wave from a 220 V 50Hz ac in my house?

Yes. It will be rather low energy and extra low frequency, but yes.

 

[anorlunda]

So, I can create and detect em wave from a 220 V 50Hz ac in my house?

https://en.wikipedia.org/wiki/Spark-gap_transmitter

But don't try it at home. Since the 1800s, we have passed many laws restricting use of the radio spectrum. If you made a spark gap transmitter without a license today, it would interfere with communications in your locality. You could be arrested.

 

[hutchphd]

The spark provides a voltage dependent switch which facilitates the resonant excitation of the circuit to oscillate at its natural frequencies. Because the emission of radiated energy depends upon acceleration of electrons, higher frequency circuits radiate more effectively.
There is also a difference between "near field" and "far field" effects. The two coils of a transformer communicate without direct connection but we do not usually call this an EM wave. Only the "far field" communication (~>10 wavelengths≡ c/freq ) is called radio waves. For 60 Hz the wavelength is 3000 miles, so call it what you will at your house!

https://en.wikipedia.org/wiki/Spark-gap_transmitter

.OOPS I see I was pre-empted!

 

[Cutter Ketch]

A spark results in a nearly instantaneous increase in the current from nothing to a very large value. This makes a lot of EM radiation and the sharp step function means the EM radiation covers the spectrum from low frequency up to the bandwidth of the sharp edge. Essentially you get a large sharp pulse of broad spectrum radiation. Although you don’t need a spark to make EM radiation, it makes sense that this big noise maker was the first place Hertz started looking.

 

[jtbell]

applying an ac voltage should create oscillating charge in the two electrodes which should create em waves with or without a spark. Please, tell me how I'm wrong.

You are correct, in principle. In practice, however... you might try doing some research on the state of AC generation technology when Hertz did his famous experiments in the late 1880s, in particular the frequencies that could be generated. Then consider how one might have detected the EM waves generated by those ACs.

 

[tech99]

A spark results in a nearly instantaneous increase in the current from nothing to a very large value. This makes a lot of EM radiation and the sharp step function means the EM radiation covers the spectrum from low frequency up to the bandwidth of the sharp edge. Essentially you get a large sharp pulse of broad spectrum radiation. Although you don’t need a spark to make EM radiation, it makes sense that this big noise maker was the first place Hertz started looking.

In the case of a spark transmitter, however, we use a resonant circuit. The radiation then consists of a damped sine wave and you can see this on an oscilloscope. The spectrum is relatively narrow, and is that of the resonant circuit. The reason the energy spreads out from a single frequency for a spark transmitter seems to be that the sine wave is distorted by virtue of being damped, but in addition, every spark initiates a wave with a random phase. On an oscilloscope you see many sine waves superimposed.
The operation of a spark transmitter can be observed in the laboratory very easily by using a relay as a buzzer with an LC circuit connected to the contacts.
There is no hope of obtaining measurable radiation at 50Hz in a practical situation.

 

[nsaspook]

Radiation at close to power line frequencies is impractical for all but M.A.D. countries.
http://www.vlf.it/zevs/zevs.htm

 

[zoki85]

Radiation at close to power line frequencies is impractical for all but M.A.D. countries.
http://www.vlf.it/zevs/zevs.htm

Nice page. That's a ground dipole
Operating principle has much in common with electrically small (in terms of a wave lenght) magnetic loop antenna. The system is certainly very poor radiator and near field effects are probably dominant up to several hundred miles away from the transmitter's location.

In the case of a spark transmitter, however, we use a resonant circuit. The radiation then consists of a damped sine wave and you can see this on an oscilloscope. The spectrum is relatively narrow, and is that of the resonant circuit. The reason the energy spreads out from a single frequency for a spark transmitter seems to be that the sine wave is distorted by virtue of being damped, but in addition, every spark initiates a wave with a random phase. On an oscilloscope you see many sine waves superimposed.
The operation of a spark transmitter can be observed in the laboratory very easily by using a relay as a buzzer with an LC circuit connected to the contacts.

Spectrum of a spark gap transmitter is a very different animal in comparison with simple spark gap Hertz radiator.

 

[tech99]

Nice page. That's a ground dipole
Operating principle has much in common with electrically small (in terms of a wave lenght) magnetic loop antenna. The system is certainly very poor radiator and near field effects are probably dominant up to several hundred miles away from the transmitter's location.Spectrum of a spark gap transmitter is a very different animal in comparison with simple spark gap Hertz radiator.

Hertz used a dipole with end loading plates, so it was a resonant device having a Q of perhaps 20. So I would not expect the action to be different from a spark transmitter, except that there would be radiation at the overtone frequencies of the antenna (3f. 5f etc).

 

[zoki85]

Coupling between two circuits, spark quenching action, spark succession rate...all that make the difference

 

[sophiecentaur]

So, I can create and detect em wave from a 220 V 50Hz ac in my house?

A long time ago, I was given an ancient Hoover vacuum cleaner and, when I used it, the guy next door couldn't listen to FM radio or MF because of the radiated interference from the sparks in the commutator of the motor (240V). (He connected the interference with the sound of the vacuuming.) There was no interference filter on it. I fitted a couple of RF chokes and Capacitors and was friends with him again. It's a requirement nowadays that all domestic equipment satisfies Interference Requirements and brush motors are a problem. Solid state power control circuits are potentially as bad because they switch current on and off so fast that the induced voltage spikes cause significant radiated power if they are not filtered.

The mains lead carries a lot of Radio Frequency current due to sparks and switching spikes and can act as a very good 'long wire' aerial for launching the wave! Easy to detect this with an MF radio receiver, held close to some electrical appliances. tAs has already been stated, it's illegal to produce more than a very low level of interference at RF. If you want to communicate across short distances and you don't want to use a 'bought' walkout talkie, you could investigate Optical methods and the regulations are a lot slacker. Even a simple LED, fed directly from an audio amp can carry a (hideously distorted) sound signal which can be detected with an optical sensor, connected to another audio amp. Not 'easy listening' but fairly easy to achieve.

Motor car (petrol engine) ignition systems also produce detectable interference - although the regulations forced designers to have very tight interference filtering (for the car radio as much as the nearby houses).

But to return to your question, a spark is very visible so that implies that optical EM frequencies are also produced due to the ionisation of the metal and gases between contacts. The Energy of Optical photons is only a Volt or two so the Mains supply has more than enough Potential. Interrupting a large current can also induce even greater Voltage spikes which can also produce UV frequencies. Detectable by eye and camera.

 

[anorlunda]

If you have the wealth of a rich nation, you could do it at 50 Hz.

https://en.wikipedia.org/wiki/Communication_with_submarines#Very_low_frequency

Extremely low frequency[edit]

\
1982 aerial view of the US Navy Clam Lake, Wisconsin ELF facility.
Electromagnetic waves in the ELF and SLF frequency ranges (3–300 Hz) can penetrate seawater to depths of hundreds of meters, allowing signals to be sent to submarines at their operating depths. Building an ELF transmitter is a formidable challenge, as they have to work at incredibly long wavelengths: The U.S. Navy's system, Seafarer, which was a variant of a larger system proposed under codename Project Sanguine,[1] operated at 76 Hertz,[2] the Soviet/Russian system (called ZEVS) at 82 Hertz.[3] The latter corresponds to a wavelength of 3,656.0 kilometres. That is more than a quarter of the Earth's diameter. Obviously, the usual half-wavelength dipole antenna cannot be feasibly constructed.

Instead, someone who wishes to construct such a facility has to find an area with very low ground conductivity (a requirement opposite to usual radio transmitter sites), bury two huge electrodes in the ground at different sites, and then feed lines to them from a station in the middle, in the form of wires on poles. Although other separations are possible, the distance used by the ZEVS transmitter located near Murmansk is 60 kilometres (37 miles). As the ground conductivity is poor, the current between the electrodes will penetrate deep into the Earth, essentially using a large part of the globe as an antenna. The antenna length in Republic, Michigan, was approximately 52 kilometers (32 mi). The antenna is very inefficient. To drive it, a dedicated power plant seems to be required, although the power emitted as radiation is only a few watts. Its transmission can be received virtually anywhere. A station in Antarctica at 78° S 167° W detected transmission when the Soviet Navy put their ZEVS antenna into operation.[3]

Due to the technical difficulty of building an ELF transmitter, the U.S., China[4], Russia, and India are the only nations known to have constructed ELF communication facilities. Until it was dismantled in late September 2004, the American Seafarer, later called Project ELF system (76 Hz), consisted of two antennas, located at Clam Lake, Wisconsin (since 1977), and at Republic, Michigan, in the Upper Peninsula (since 1980). The Russian antenna (ZEVS, 82 Hz) is installed at the Kola Peninsula near Murmansk. It was noticed in the West in the early 1990s. The Indian Navy has an operational VLF communication facility at the INS Kattabomman naval base to communicate with its Arihant class and Akula class submarines.[5][6][7] China on the other hand has recently constructed the world's largest ELF facility roughly the size of NYC in order to communicate with its submarine forces without having them to surface.[8]

 

[zoki85]

I would think frequencies like 50 or 60 Hz are to be avoided due to higher possibility of interference

 

[sophiecentaur]

If you have the wealth of a rich nation, you could do it at 50 Hz.

At 50Hz, the transmissible data rate would be very low but a spark would generate many more useful frequencies providing a much more useful information capacity..

 

[anorlunda]

At 50Hz, the transmissible data rate would be very low but a spark would generate many more useful frequencies providing a much more useful information capacity..

If I remember correctly, the data rate was really low. But the need for bandwidth required for submarine communication is not high. The critical message need only be 1 bit long.

0=don't fire
1=fire

Hopefully, some error correcting bits would be added .

I remember once disembarking a plane at the airport. A sign over the door said, "your baggage will be at station B" I thought, "Wow how useful that information is although only 2 or 3 bits long." My point is that bandwidth does not always equal value.

 

[sophiecentaur]

. . . And most of the traffic on broadband, we could do without.