TESLA RESEARCH

Low Power Magnifier Coil

During the course of Tesla's investigations into high voltage - high frequency currents, he discovered the principle of free resonance, where a coil was found to generate high voltage discharges while in close proximity to powered resonant transformer. Tesla realised that this remotely energised coil was drawing its power from the high frequency currents conducting through the ground from the powered resonant transformer. Not only did this discovery spur on the great inventor's ideas about wireless transmission of electrical power, but also brought about the invention of the magnifier.
The magnifier is made up of two main parts, a powered high frequency transformer that has large coupling between its primary and secondary, which is operated outside of any resonant modes. The second part is simply a tesla coil secondary circuit, that is a coil of many turns topped with an isotropic capacity such as a sphere or torroid. This will have its own natural frequency of free resonance, and it is this frequency at which the high frequency transformer is made to resonate at. Due to their being no magnetic coupling between the power source and the resonant component of the system, the magnifier proves to be of much greater efficiency than the standard tesla coil. In a standard tesla coil, some of the energy stored in the secondary circuit can make its way back into the primary circuit resulting in loss of energy. This is usually counteracted through spark-gap design; however the magnifier system deals with this, and other problems, more efficiently by doing away with magnetic coupling altogether.

When technical descriptions first appeared explaining magnifier operation, they tended to be complicated and a little confusing. I, like many, discovered how to build a magnifier purely by accident. All I really knew was that a magnifier looked like a tesla coil with another coil stuck on top instead of a torroid or sphere. So while setting up my own coil I gave it a try and was amazed at the sudden appearance of heavy corona. After a little tinkering I managed to assemble a fairly good tabletop magnifier, which I then tried back calculating to find out what was happening.
The whole system was powered by a neon sign transformer rated at 125VA with a voltage output of 5kV. A bank of twenty 10nF/7.5kV capacitors wired in series-parallel gave a total capacity of around 13nF rated at 30kV; this formed the primary circuit capacitor. With a peak supply voltage of approximately 7kV, this capacitor will store 325mJ of energy. Assuming the spark gap fires correctly at 100 times a second (gap firing on every half cycle of the 50Hz line frequency), then the actual power consumed will be 32.5W, around a quarter of the transformers maximum power capacity of 125VA. The primary coil of the high frequency (HF) transformer was constructed from an upturned flowerpot, with small wooden pieces glued to the side to hold the coil. The coil its self consisted of twelve turns of 16swg tinned copper wire, perhaps not the best choice.

The spark gap resembled more the quenched type, offering some heat dissipation through its construction. It was in fact made from two Victorian-style doorknobs facing each other. This worked O.K, but the brass surfaces became easily tarnished, requiring a polish after a few minutes of operation. This was probably the greatest fault of this magnifier system - next time a rotating gap shall be used!
The secondary coil of the high frequency transformer was wound on a short length of one and half inch diameter PVC pipe. 0.212mm diameter enamelled copper wire was wound onto this form giving a total coil length of around six inches, resulting in approximately 700 turns. Using the Wheeler formula, the inductance of this coil was found to be 4.75mH. The base of this coil was connected to the foil ground plane.
When the whole circuit was powered up it was possible to draw a small blue-white spark from the top of the secondary coil, with the tip of a screwdriver. This was done through the screwdriver handle and a 3kV insulated glove that I wore during this test. I had to hold the screwdriver handle by its very end so as to avoid being prickled by tiny streamers, which managed to penetrate the glove.

The extra coil or resonator was wound using the same 0.212mm enamelled wire. This was wound onto a length of two and a half inches diameter PVC piping. The total coil length was eight and a half inches, and the total number of turns was approximately 1000. The calculated inductance was work out to be 17mH. A metal sphere was placed on top of this coil to act as an isotropic capacity or top load. It had a diameter of 100cm or four inches, which gave it a theoretical electrical capacity of approximately 6pF. The self-capacity of the coil was calculated using Medhurst and was found to be approximately 4pF. Combined with the inductance of the extra coil, the resonant frequency of the coil-sphere was calculated to be around 340kHz.
When connected to the HF transformer and after some tuning, streamer broke out from the metal sphere atop of the resonator or extra coil. To a fixed point connected to ground, sparks averaging six inches were noted, however streamers into free space have on occasion been twice as long.

When a measurement was taken of the primary's inductance at optimum tuning, it was found to be 7.5mH. This, after calculation, gives a primary frequency of approximately 500kHz. This is somewhat out of tune with the predicted 340kHz resonant frequency of the extra coil and top load. It was concluded that the positioning of the sphere (directly on top of the extra coil) might have resulted in its isotropic capacity being greatly reduced. This in turn would heighten the frequency of resonance for the coil-sphere circuit. The self-resonance of the extra coil was calculated to be around 600kHz, this frequency was then only slightly lowered by the top load.

Admittedly this set-up was not built to last and many corners were cut. However the results were quite good and clearly show the magnifying effect. If you have any comments or suggestions, then please email me.