Tesla's "COIL FOR ELECTRO-MAGNETS".

[MileHigh]

You can see when you look at the drawing that there are four layers of band-iron formed into coils that are stacked one on top of the other.  There are some nice supplementary drawings if you follow the patent links.

All four layers form a giant ampere-turn magnetic field generator that work together.  You can see how the metal casing of the electromagnet takes the "top" flux and channels it to the center and "points" it downwards.  The same "top" flux is also channeled to the outer ring of the casing and is also "pointed" downwards.  So Gyula's "magnetic footprint" drawing is wrong.   If the center circle of his drawing is red, then the outer ring that he has marked in blue is also actually red.  Following Gyula's convention where blue represents the opposite polarity, then what Gyula shows as white should be blue.  In summary, at the inside you have the red circle, then you have a blue ring around that, and on the outside you have a thin red ring.  That's how the electromagnet is designed.

In looking at the other drawings, the blue area (my definition) seems to be recessed.  That makes the recessed blue area a kind of "suction zone" for scrap metal when the electromagnet is energized.

I will repeat again, to make a connection between Tesla's patent and the two electromagnet patents shown here is nonsensical.  Talk about minuscule inter-layer capacitance in this brute force application makes no sense at all.

MileHigh

[sparks]

   The old magnets in junkyards have to let go of the stuff.  They used at least two windings.  Like an automotive starter solenoid.  One high current to polarize the load steel.  One low current to maintain the stuff on the magnet while it was transported.    Then when you want to drop the stuff the holding current winding is opened.  The kick voltage of the high resistance winding goes sky high so the switching devices used mercury plungers I believe and dumped the kick energy into resistor banks.  This is from memory of a brief read 30 years ago so take it for what it's worth.
  IMHO a tesla pancake coil is nothing but a piece of coiled coax.  If you take a piece of coax and hit it with RF to INCREASE the standing wave ratio  (reflected signal)  soon you develop high voltage relavent to nodes and antinodes within the coax.  If your node and antinode develop at terminals then you get what Tesla was after.  We know that electromagnetic waves carry energy so when they stand and are constructive the pancake coil acts as an energy storage unit.

[gyulasun]

Hi sparks,

I agree with you, in the Tesla pancake coil as defined in his Coil for Electromagnet patent, Figure 2, the two conductors (in very close proximity but still isolated from each other) constitute a transmission line just like the coax cable that also includes two isolated conductors.  Ham radio operators do use so called coax trap coils to get frequency selective switches in their antennas to have multiband operation in the short wave bands. The coax traps have many series and parallel resonances as per their impedance curve shows in the function of frequency: http://vk1od.net/antenna/coaxtrap/coaxtrap10.png  from this link: http://vk1od.net/antenna/coaxtrap/ 
It is sure that Tesla was aware of how the impedance of his two conductor coil changed when connected the two isolated conductors in series because he wrote that at resonance the input AC current finds no impedance other than the conductors resistance, this way the usual and inherent coil propety to resist initial current flow is solved. 

However, in the junkyard electromagnets (as per the two patents described in my previous post above) the pancake coils used single conductor and no any mention of using two conductors as per Tesla's  Fig.2 defined.  So what you wrote energy-wise for the coiled coax (which I agree with)  is not valid for a single conductor pancake coil.

Gyula

[synchro1]

   The old magnets in junkyards have to let go of the stuff.  They used at least two windings.  Like an automotive starter solenoid.  One high current to polarize the load steel.  One low current to maintain the stuff on the magnet while it was transported.    Then when you want to drop the stuff the holding current winding is opened.  The kick voltage of the high resistance winding goes sky high so the switching devices used mercury plungers I believe and dumped the kick energy into resistor banks.  This is from memory of a brief read 30 years ago so take it for what it's worth.
  IMHO a tesla pancake coil is nothing but a piece of coiled coax.  If you take a piece of coax and hit it with RF to INCREASE the standing wave ratio  (reflected signal)  soon you develop high voltage relavent to nodes and antinodes within the coax.  If your node and antinode develop at terminals then you get what Tesla was after.  We know that electromagnetic waves carry energy so when they stand and are constructive the pancake coil acts as an energy storage unit.

Thanks Sparks. The high voltage polarization you speak of is also refered to as: "Impulse Magnetization". Those old fashioned magnets are long gone. Consider what the time frame did for the field of Aviation, from Kitty Hawk to the Mars Rover. It's hard at first for us to understand the Wright brother's method of wing warping to gain directional control, even though wing flaps are commonplce today on aircraft.

The argument arose about the differences between SBC and single wire coils. Both coils are used to "Impulse Magnetize" but they work differently. The SB Pancake directs the pulse to align electron spin in a substrate while the single wire wrap coil polarizes.

[gyulasun]

Layering counterwound pancakes together cause them to share the same kind of characteristics found in the serial bifilar pancake.

Sorry, it is not correct.  The facing pancake surfaces are not comparable for the two cases, that is why I disagree.  See also Magluvin answer here http://www.overunity.com/13460/teslas-coil-for-electro-magnets/msg361720/#msg361720

Gyula