# [TCML] Spark Length Measurement Techniques

bartb bartb at classictesla.com
Mon May 18 19:34:31 MDT 2009

```Hi All,

As "the current record holders" thread was wrapped up, it does spin off
to spark length measurements. As a few are using photographs for this
purpose, I couldn't help but comment.
> Herr Zapp wrote:
> Interpolating the dimensions of or distances between objects shown in
> a photograph is a simple and well-established technique known as
> photogrammetry. One application of photogrammetry is to determine
> distances between various objects in a photo  based on the known
> dimensions of a reference object contained in the photograph.

I certainly understand that references are needed and this can be done
to a degree, but with sparks, this gets tricky. In order for the spark
to be accurately measured via a photograph, the spark "must be"
perpendicular to the camera. Even a few degrees into the Z axis, and the
"object of reference and known dimensions" must be reduced in size by
the angle. All sparks that have any angle to the camera from the
perpendicular will always make the sparks shorter than they actually
are. 13M did not have a single perpendicular arc in the photo taken. The
only arc close is the one on the right that issues from the back and
curves around the coil forward. If you measure with some known reference
object, you still completely miss the Z axis length. I don't believe
this to be the claim photo as I've already stated in the other thread,
but represents my point regarding Z-axis. Not only with the angle the
spark issues, but also it's curvature in that direction.

The best way of course is to never rely on a photo as the measurement,
but rather a measured "target" the coil arcs to within a photo. Spark
length claims without a photo are not proof, but this doesn't mean the
claim is false. It simply means you must take the claimers words with
faith that it is true or not.

My little coil 4.5"D x 21.165"H max spark length is 55" to a target. At
my front page of my website is a photo of this coil and an arc (top right).
http://www.classictesla.com/

The two toroids edge to edge nearest distance is 55" measured with a
tape measure. If you were to use a little stick man or other 2D object,
what would you estimate the distance to be? For reference, the toroid on
the little coil is 6.25" x 21".