Primary field strength
From: chris.swinson [SMTP:chris.swinson-at-zetnet.co.uk]
Sent: Sunday, August 16, 1998 3:48 PM
To: Tesla List
Subject: Re: Primary field strength
Thanks to all who set me straight on BTW, Rather good really.
Right, thanks to Bert for you input :-)
I'll try to explain myself a little better this time.
I took off my 36" secondary and just had the primary on its own. My
capacitor was taken out as well. I have 2 caps, One 11.45nF, and ther other
6.4nF. Does not matter which for this experiment. Mind you people could
argue over my testing of this but I thinks its a good way to do it....
Right I have a spark gap, a RQ type ( Yes I know :-) ), My First test was
set for 5 gaps. Thats about 5mm. The NST just got across them all and as
expected they were week febble purple sparks.
I wanted to test how big the field strengh was to see if I could get away
with a larger secondary coil. My old coil was 24" in hight, and my new one
was 36" in hight, so I just wondered how much bigger I could make it.
I got hold of a "wire finder" , you know, those hand held things which allow
you to trace wires in walls. Well you get the idea.
I set the sensitivity so that I could get it right next to the primary
not turned on ) without it beeping. As It got near the coil ( only about
1/4" ) it beebed. So I set it to about that.
I then turn on my NST, When the Wire finder ( WF ) got near the primary it
sounded. I then moved slowly straight up to see how far the filed would go.
At about 40" the thing stopped sounding. So I now knew that I could just
get away with a 40" secondary.
I then added a cap ( don't matter which one ) and set the gap to 1mm. note
I did not have the secondary connected in ANY test.
As expected there was bright flash's across the gap. I put the WF next to
the primary and it sounded, I moved up, Still sounding, I moved up more,
Still sounding. I went into another room, Still sounding, Well you get the
so if the field is that big, I could easily make a 200" secondary, as it
appears the field is huge when the caps are included. Ok, I could not do
this as I only have 80" hight in my room. But you see what I'm getting at.
This was only 1 gap as well, If I used all 5 gaps , the field would probably
go 100's of yards away. So what stopping me from building a bigger
The only limit I see is the ringing of the coil. The field would get
smaller and smaller as the charge in the cap would decrease.
It looks like to me, The secondary used the RMS power of the primary, You
could ( to me ) build a coil 200" high, but as the power is dropping the
secondary would not respond to the last half of the charge cycle in the cap
( did that make sence ).
So Even though I'd still be pulsing the secondary at say 500Khz, it would
only pulse it for 50% of the time as it would be out of range when the power
dropped. To me this wouln't matter as you'd still appear to get constant
sparks even though the coil would miss 50% of the resonant cycle. This is
only my opinion of this, I'm not saying its true at all.
Thats why I'm asking you lot to see what you think.
All the beST,
www.users.globalnet.co.uk/~mega01/ <--- moved site to...
From: Tesla List <tesla-at-pupman-dot-com>
To: 'Tesla List' <tesla-at-pupman-dot-com>
Date: 16 August 1998 17:41
Subject: Primary field strength
>From: Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
>Sent: Saturday, August 15, 1998 12:10 PM
>To: Tesla List
>Subject: Re: Primary field strength
>I'm not exactly clear what field you're describing as being 38" high
>with a 11.45nF cap), filling a room (with a 6.4nF cap), and how you
>measured it. Can you be a bit more specific?
>The length of a secondary is not very critical as long as it's at least
>long enough to withstand the voltage stress it'll see during operation.
>However, even with a shorter secondary, most of the magnetic coupling
>between the primary and secondary takes place at the bottom-most portion
>of the secondary. In fact, 80% or more of the primary's magnetic field
>is NOT coupled to the secondary coil... by design. And, increasing the
>length of your secondary coil while leaving your primary unchanged will
>actually DECREASE the amount of coupling. Paradoxically, in a system
>with a poorly quenching sparkgap, this can actually increase spark
>length.. and it fixes a symptom of coil inefficiency but not the cause.
>The ratio of the length versus height is also not especially critical,
>and Tesla Coils can be made to operate with long "candlestick"
>secondaries, although not as efficiently. In order to get the best
>secondary efficiency, the ratio of the secondary's inductance (actually
>it's inductive reactance) versus its effective resistance should be kept
>high in order to minimize losses. Tesla used large diameter coils to
>help prevent corona breakdown and to provide as much inductance as
>possible while keeping coil resistance and distributed capacitance to a
>minimum. It's a case of trying to balance conflicting objectives in
>order to maximize overall performance. Long, thin, coils tend to have
>lower Q, and thus lower operating efficiencies.
>Ultimately, spark length is primarily a function input power and coil
>efficiency. An 8 KV 30 MA power source (240 VA) will not create 72"
>streamers irrespective of coilform length - it simply takes more power
>to achieve this level of performance. You'll need 1-2 KVA to reach this,
>even in a well-designed system.
>You also didn't indicate the diameter and wiregauge of your secondary,
>but it sounds like it's about 3" and 21 AWG based upon the inductances
>you've quoted. As voltage levels rise, you also need to increase the
>coilform diameter to prevent premature breakout from the coilform
>itself. Since you have the rest of the system constructed, try
>constructing a couple of experimental coils using say 60" or 80"
>lengths, see if the improvement you observed continues.
>And, please let us know the results.
>And BTW, BTW = By The Way... :^)
>Safe coilin' to you!
>-- Bert --
>Tesla List wrote:
>> From: chris.swinson [SMTP:chris.swinson-at-zetnet.co.uk]
>> Sent: Friday, August 14, 1998 3:20 PM
>> To: tesla list
>> Subject: Primary field strength
>> Hi all,
>> I discovered today that my coil's field strength drops off at about 38".
>> used a 8Kv 30ma NST. My secondary ( 38" ) and cap ( 11.45nf ) were not
>> at first. This gave me a clear indication that the field rose to a hight
>> 38". I could gain about another 2" spark if a increased the coil's
>> but thats not the issue here.
>> When I tried a 6.4nf cap, with only 1mm spark gap, The field was so big
>> it filled the whole room. If I had used a 5mm gap ( about max ) Wouldn't
>> be possible to have a secondary say 200" in hight and still power it off
>> 8Kv 30ma source.
>> The secondary would still be in the field of the primary as the cap
>> high current boost each cycle.
>> But even so, why am I told not to go over 24" for my secondary height ?
>> The only problem I can see is the resistance of the secondary would be
>> due to the extra wire, but I can double wind my coil to get around this.
>> in fact I could get about a 72" spark off a 200" coil, with the power
>> being only 8Kv -at- 30ma.
>> I know Tesla used large inductive coils, perhaps he did this to use
>> caps and get better effieciency.
>> The only other drawback being the frequency would drop due to the high
>> inductance, but my specualted coil at 200" high, would only give 44mH
>> inducatnce anyway. My 36" coil is only about 8mH anyway. So the way I
>> it, its a win-win situation.
>> Can anyone confirm this or have anything to comment on ?
>> By the way, I'm a bit think, so whats BTW stand for ? Its driving me mad
>> All the beST,
>> www.users.globalnet.co.uk/~mega01/ <--- moved site to...