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Re: Variable Inductor

 * Original msg to: Rwall-at-ix-dot-netcom-dot-com
 * Carbons sent to: usa-tesla-at-usa-dot-net

Quoting Richard Wayne Wall <rwall-at-IX.NETCOM.COM>:

 > Richard Wayne Wall wrote:

 >> I've noticed roller inductors used in radio transmitters      
 >> advertised in the Fair Radio Sales catalog... Has anyone any  
 >> experience putting one in the primary circuit of a Tesla      
 >> coil?  Seems like a good safe fast way to fine tune
 >> a coil.  May be eliminating multiple taps on the primary.

Quoting Scott Myers:

 > Bottom line:  Keep every tiny percentage of the total          
 > inductance of your tank circuit right in the primary windings. 
 > All other connections in the tank circuit should be as short   
 > as possible.

 RW> Scott,

 RW> Following your logic then adding any off axis inductance     
 RW> will reduce primary efficiency including chokes and ferrite  
 RW> cores that are added to inhibit kickback to the transformer. 
 RW> Should they be elininated also? 

You are getting off on the wrong foot here. The key words are
"Tank Circuit". This is specifically referring to the oscillating
circuit between the primary coil "L1", the tank circuit capacitor
"C1", inclusive of the main system spark gap G1. This forms the
classic LC oscillator loop with the gap G1 as the oscillator
switch.

The power supply and feed lines are just that, power supply and
feed lines. The off axis inductance of the RF choke coils, and
indeed the step-up transformer secondaries are not part of the LC
tank circuit. The lines between these two high voltage circuits
should be clearly drawn, even to the gauge of wiring: the HV
transformer coils, choke leads and supply lines need only be
wired with small gauge conductors; the current flow through these
sections of the circuit is usually measured in milliamperes.
The tank circuit, or LC oscillator proper is wired with heavy
conductor; flat strap, copper tubing, or heavy battery type cable 
being typical. This is a high current tuned circuit, with peak
currents of dozens, even hundreds of amps and peak powers of
hundreds of killowatts, even megawatts, easily obtainable.

I have extensive experience in tuning these oscillators using
off-axis inductance, employed in exactly the same way as Tesla
did in Colorado Springs, and as is used in some radio circuits.

Tesla was forced to use off-axis inductance at Colorado Springs
because he could not shorten his primary, and could only lengthen
it one turn at a time. Even tapping such a large coil; the tap
lead back to the tank circuit utilities (C1 & G1) acts exactly
like a primary turn. In this instance tapping is ineffective.
Tesla had an adjustable capacitance with salt water tanks, which
I have covered in detail previously while discussing salt water
capacitors. He was able to tune the tank circuit in most
instances by varying the value of C1; until he set the infamous
Westinghouse custom transformer to the highest voltage settings.
With the tank circuit voltages run to maximum, Tesla had to put
all of his capacitance is series in order for the bottles to
withstand the electrical pressure while still meeting the tank
circuit requirements. With all of the capacitance in use he was
forced to turn to the variable inductor to tune the tank circuit.

So, in effect there are times when using an off-axis inductance
to tune the tank circuit is OK.

The problem is a little different on high performance coil
systems that are physically small. In this case the off-axis
inductance presents a real waste of energy when the tank circuit
can be tuned by tapping at different locations on primary coil.

Richard Quick


... If all else fails... Throw another megavolt across it!
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