Re: straight core transformers and chokes

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

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Dave,

What you describe is a common induction coil of the era.

http://home.datacomm.ch/k.schraner/induction_coils.htm

http://www.sparkmuseum-dot-com/INDUCT.HTM

A modern ignition coil or two for a car will easily outperform such a 
device now-a-days...  Really not a Tesla coil at all, but rather a high 
voltage induction coil.

If you really want to make one for the sake of nostalgia, they are really a 
bit complicated to get just right.  What you need is the book "Design and 
Construction of Induction Coils" by A. Frederick Collins (1908) which has 
been reprinted by Lindsay publications:

http://www.lindsaybks-dot-com/

You may have to call them since I don't see it right off in the catalog 
(not the most well organized publication ;-)).  You can actually buy 
annealed wire right off the shelf rather than doing the back yard blast 
furnace thing.  Or, you could just get modern transformer steel ;-))

PV also has a bunch of books:

http://www.arcsandsparks-dot-com/reprintpage.html

The key is getting a good long spark without blowing them up.  If they are 
not made right they will arc out and fail.  E-bay has all the wire you 
could ask for in smaller gauges.

Cheers,

         Terry


At 11:21 AM 9/2/2003 -0400, you wrote:
>I am contemplating the construction of a straight core transformer. I read 
>an old Tesla coil how-to book from the 20's that was on microfilm in the 
>New York public library a long time ago. My hand written notes from 20 
>years ago are as follows:
>
>"Pieces of 18" long #20 or 22 B&S gauge iron wire, stuff as many as will 
>fit into an iron pipe less than 2" in diameter. Throw into a coal fire and 
>leave until it dies out. Remove iron wire, clean, and coat individually 
>with shellac. Push back into pipe, then push out the bundle slowly, wrap 
>with cotton bandage, 1" wide, and coat with shellac.
>
>The builder was to then create a number of thin wooden discs 6" in 
>diameter, with a hole cut in the center large enough to slide them onto 
>the bundle of iron wire. The primary and secondary windings would be wound 
>between the disks. The disks are to be 3 1/4" apart along the bundle of 
>wire. The transformer would have 4 sections, each 3 1/4" wide, with a 1" 
>gap in the center, between the 2 primary windings and the 2 secondary 
>windings. Just imagine 4 donuts stacked on a paper towel roll and you get 
>what this is to look like.
>
>"Primary is wound in 2 sections with #12 B&S double cotton covered wire, 2 
>1/2 lbs. Wind 60 layers, with a turn of paper between each layer."
>
>The next section is made essentially the same. For some reason, I didn't 
>write down the gauge of the secondary wire, or how many turns were 
>required. I do remember that it was a lot of layers.
>The book claimed that a straight core was superior to a closed iron core. 
>This, I wrote down word for word...
>
>"A straight core is always best to use; for on the fall of the current 
>from its maximum value to zero, the magnetic flux falls from its maximum 
>value, not to zero, but to a value which depends on its residual 
>magnetism. The residual magnetism in an open circuit is much less than in 
>a closed magnetic circuit so that when the current suddenly becomes zero, 
>the magnetic flux drops lower in an open circuit than in a closed one. As 
>the electromotive force in the secondary is proportional to the fall in 
>the magnetic field, it is greater with a straight core than with a closed 
>circuit of iron"
>
>So here's my plan.... I see that Mcmaster Carr sells iron wire, in 1 foot 
>lengths, or rolls. It is high carbon wire. I want to take a bundle, 
>varnish each piece, stuff it into a thin walled phenolic tube, cut the 
>wooden disks which will divide the primary and secondary windings, and 
>start winding a transformer.
>
>I would like to hear from anyone that has knowledge of transformer design 
>parameters to evaluate this, and submit your thoughts on this type of 
>transformer before I start building it. Any guess on the gauge of wire for 
>the secondary? How many turns?
>Does anyone recognize this book? It was titled something like "How to 
>build and test a complete Tesla coil" It also described the construction 
>of a high voltage rectifier by filling a fish tank with battery acid and 
>sinking an array of plates made of iron and aluminum,  similarly to a car 
>battery.
>
>Thanks to all who reply,
>Dave Goodfellow
>