Tesla’s best-known invention takes the spark-gap oscillator and uses it to vibrate
vigorously a coil consisting of few turns of heavy conductor. Inside of this
primary coil sits another secondary coil with hundreds of turns of slender wire.
In the Tesla coil there is no iron core as in the conventional step-up transformer,
and this air-core transformer differs radically in other ways.
Recounting the birth of this invention, Tesla wrote, "Each time the condenser was
discharged the current would quiver in the primary wire and induce corresponding
oscillations in the secondary. Thus, a transformer or induction coil on new principles
was evolved Electrical effects of any desired character and of intensifies undreamed of
before are now easily producible by perfected apparatus of this kind." Elsewhere Tesla
wrote, "There is practically no limit to the power of an oscillator."
The conventional step-up transformer (short primary winding, long secondary on an iron core)
boosts voltage at the expense of amperage. This is not true of Tesla's transformer.
There is a real gain in power.
Writing of the powerful coils he experimented with at his Colorado Springs lab, coils with
outputs in excess of 12 million volts, Tesla wrote, "It was a revelation to myself to find
out that ... a single powerful streamer breaking out from a well insulated terminal may
easily convey a current of several hundred amperes! The general impression is that the
current in such a streamer is small."
how it works
A tesla-coil secondary has its own particular electrical character determined in part by the
length of that slender coiled wire. Like a guitar string of a particular length, it wants to
vibrate at a particular frequency. The secondary is inductively plucked by the primary coil.
The primary circuit consists of a pulsating high-voltage source (a generator or conventional
step-up transformer), a capacitor, a spark gap, and the primary coil itself. This circuit
must be designed so that it vibrates at a frequency compatible with the frequency at which
the secondary wants to vibrate.
The primary circuit's frequency is determined by the frequency and voltage of the source,
the capacity of the capacitor, the setting of the spark gap, and the character of the primary
coil, determined in part by the length of its winding. Now when all these primary-circuit
components are tuned to work in harmony with each other, and the circuits resulting frequency
is right for plucking the secondary in a compatible rhythmic manner, the secondary becomes at
its terminal end maximally excited and develops huge electrical potentials, which if not put
to work, boil off as a corona of bluish light or as sparks and streamers that jump to nearby
conductors with crackling reports.
Unlike the conventional iron-core step-up transformer, whose core has the effect of damping
vibrations, the secondary of the Tesla transformer is relatively free to swing unchecked.
The pulsings from the primary coil have the effect of pushing a child in a swing. If it's done
in a rhythmic manner at just the right moment at the end of a cycle, the swing will oscillate up
to great heights. Similarly, with the right timing, the electrical vibration of the secondary
can be made to swing up to tremendous amplitudes, voltages in the millions. This is the power
Also in this chapter:
a new power system
THE LOST INVENTIONS OF NIKOLA TESLA
by George Trinkaus
republished with permission of the author and publisher
High Voltage Press
4326 S,E. Woodstock, #489
Portland, OR 97206 USA