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Re: 7.1Hz, how the heck did Tesla succeed?

Original poster: stork <stork@xxxxxxxxxxxxxxxxxxx>


Steve,

What is the location and in what element do these displacement currents occur?

Like I already said, they are a mathematical fiction to account for the (experimentally discovered) fact that electric charges exert forces on each other over a distance.

For the sake of argument, we can use displacement current in the same context Maxwell used. I agree displacement current is a mathmatical fiction as well as a real world fiction. But, as Maxwell defined displacement currents, they are real currents measured in amperes. And, as such, should be experimentally measured as any other current measured in amperes. I agree that all charges both positive and negative exert forces on all other charges.

All that really happens is: When Tesla fires up his transmitter, electrons come spewing out the bottom of the coil into the ground, all ready to whiz to the ends of the earth at light speed, lighting lamps, driving electric clocks, and faxing the front page of the Wall Street Journal.
But, those electrons came from the topload! Since it's empty of electrons, it is a huge positive charge. Opposite charges attract, so those electrons spreading out in the ground get dragged back towards the transmitting tower, and never make it further than about a quarter mile.

Previously you said, "He (TESLA) didn't realise that displacement current cancelled out practically all the radiated power in the far field.

Two things here. You invoke far fields and radiated power in your previous writing. Where as now you now describe near fields and emission of electrons rather than radiating EM waves in the far field. Do you think electrons from the earth mediate displacement current in the radiated EM far fields? Do you think electrons mediate displacement current in a simple RC closed loop circuit driven with an AC current?

Half a cycle of oscillation later, the mirror image happens but the result is the same- The current that Tesla thought would travel throughout the whole globe, doesn't. Where is all that missing current? It looks "as if" it got sucked out of the ground, through the air, and back up to the topload!


Of course it couldn't have, because air is an insulator, hence electrons can't travel through it, and this is where the dilemma starts.

This last sentence is correct. Air is an insulator and a dielectric too. You're on the right tract.

Trying to
actually measure displacement current is like trying to tape-record the sound of one hand clapping. We don't have any instrument that can measure current in any other form than the motion of electric charges, so we can't measure displacement current except by putting out an antenna loaded with electrons and letting the field displace them through our instrument. This turns the displacement current back into ordinary conduction current, so you're not really measuring displacement current at all.
To sum up, displacement current is like Santa Claus. Technically it doesn't exist, but somehow, the milk and cookies get eaten and the Christmas gifts appear.
Curiouser and curiouser.

Steve Conner

Many posters don't understand the concept of displacement current as defined by Maxwell. Often times when they are confused or don't understand something they just grab the displacement current words and insert them erroneously. So with your indulgence, I will make a few observations regarding criteria that must apply to displacement currents. I know many will try to make their own additions, but for our purposes lets just keep it as a basic RC alternating current closed loop circuit.

1.  Displacement currents are always time varying and never DC.

2. Displacement currents only occur in insulators or dielectrics. They occur only in circuit elements like capacitors, but never in conductors of the circuit. That's conduction current and has a measurable magnetic field.

3. They are in closed loop circuits and are the same frequency, amplitude and direction as conduction currents. They usually have nearly the same phase as their antecedent conduction current, but some phase lag is always introduced by the capacitor itself. They are not tiny little currents and just hard to find. They are as big as the conduction currents.

4. No magnetic fields are experimentally measurable in dispalcement currents in dielectrics. Eventhough dispalcement currents are every bit as large as the conduction currents which produce them, magnetic fields cannot be measured even with the most sensitive instruments such as a SQUID device. There is a reason why, but I won't go into that now.

Finally electrons don't mediate dispalcement currents. EM governs current flow in the conductor up to the capacitor, then "current" becomes an electrodynamic event in the dielectric, and finally current again becomes an EM event in the conductor on the other side of the capacitor. Electrodynamic is the salient term.


stork