Re: Variable Capacitance and Inductance

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Dave,
         The quote from Tesla contains excellent examples of limited 
knowledge and/or misinterpretation of results. For example:

On 21 May 2002, at 8:03, Tesla list wrote:

> Original poster: "David Thomson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <dave-at-volantis-dot-org>
> 
> We have discussed on this list individual observations of variations in
> inductance measurements of coils.  It has brought about sometimes heated
debate
> as to whether this was a malfunction of machinery or a change of atmospheric
> conditions.  At least one person on this list has proposed to run a test over
> time to see if there were indeed a variation in inductance in a coil.
<snip to relevant passage>

> Continuing the investigation of this astonishing phenomenon I observed
that the
> ca­pacity varied with the elevation of the conducting surface above the
ground,
> and I soon ascertained the law of this variation. The capacity increased
as the
> conduct­ing surface was elevated, in open space, from one-half to
> three-quarters of 1 per cent per foot of elevation. In buildings, however, or
> near large structures, this increase often amounted to 50 per cent per
foot of
> elevation, and this alone will show to what extent many of the scientific
> experiments recorded in technical liter­ature are erroneous. In
determining the
> length of the coils or conductors such as I employ in my system of wireless
> telegraphy, for instance, the rule which I have given is, in view of the
above,
> important to observe.

The assertion that the capacitance of the terminal increases with 
height simply doesn't stand to reason (unless it was just a few 
inches from the coil to begin with - mutual shading). In elevating it 
(outdoors), he is moving it further away from ground (closer to the 
ionosphere for sure, but an absolutely trifling amount by comparison).

     What he observed was a drop in the resonant frequency of his 
coil. What he appears not to have taken into account in explaining 
this was the extra length of conductor leading to the terminal. If 
the length of a monopole antenna is increased, its resonant frequency 
drops due to the extra inductance and capacitance of the wire. 
  
     Some of the actual measurements he took appear to be grossly in 
error also. Richard Hull noted at least one pertaining to terminal 
capacitance in his "Guide to the Colorado Springs Notes". 
 
> “Far more interesting, however, for men of science is the fact I observed
> later, that the capacity undergoes an annual variation with a maximum in
> summer, and a minimum in winter. In Colorado, where I continued with improved
> methods of inves­tigations begun in New York, and where I found the rate of
> increase slightly great­er, I furthermore observed that there was a diurnal
> variation with a maximum during the night. Further, I found that sunlight
> causes a slight increase in capa­city. The moon also produces an effect,
but I
> do not attribute it to its light.

The sun undoubtably has some effects - most notably on LF/MF 
propagation due to its influence on the ionosphere (day/night and 
probably summer/winter as well when its angle to one or other of the 
poles is more oblique). I'm not sure how much of this was known or 
quantified in Tesla's time. 

>  
> 
> “The importance of these observations will be better appreciated when it is
> sta­ted that owing to these changes of a quantity supposed to be constant an
> electrical circuit does not vibrate at a uniform rate, but its rate is
modified
> in accordance with the modifications of the capacity. Thus a circuit
vibrates a
> little slower at an elevation than when at a lower level. An oscillating
> system, as used in teleg­raphy without wires, vibrates a little quicker when
> the ship gets into the harbor than when on open sea. Such a circuit
oscillates
> quicker in the winter than in the summer, though it be at the same
temperature,
> and a trifle quicker at night than in daytime, particularly if the sun is
> shining.

So water vapour in excess (clouds) appears to have had some influence 
on his measurements.
     All this to show that one must be careful in drawing conclusions.

Malcolm