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RE: static spark gaps



Original poster: Frank <fxrays@xxxxxxxxxx>

A spark gap is an electric arc and temperatures are high. Not only do you have high temperatures but corrosive gases such as ozone and others depending on the local pollution.

Depending on the type of silver, its melting temp is around 1,700+- F, tungsten is around 5,400 +-F

Unless you really cool a silver gap, it will melt and vaporize the silver at a fast rate, especially at higher currents. As the gap face becomes irregular, so does the spark performance. Anything less than pure silver, the melting temp is lower and as gap is working, it boils off the compounds of the alloy changing the performance.

Seriously, water cooling is necessary to make a high current silver gap worthwhile and you will still have a lot of maintenance between uses. The silver facing will tarnish when the gap is not in use and may need cleaning before it will work.

If you keep the power at low levels, silver could be suitable but you will probably find the gap is hard to start between uses, especially if the time "off" is long, like more that a month.

Low power gaps I have used nickle plated brass, brass and zinc. All my higher power gaps above 1/2 KW use tungsten.

I have some 1 KW + antique Tesla coils used for medical purposes and only one coil used silver with limited success. They all tried but gave up and went back to tungsten for consistent performance and life. One coil that was original with silver faces and it sparked great for about 20 sec and then the gap started sputtering and died. A quick file and it would work again for a short time. I gave up and removed the silver faces and replaced with tungsten and the gap now works every time for any amount of time for the past 20 years. These medical coils were designed to be run for hrs at a time without fussing. They had to run Xray tubes for 30 minutes to an hour continuous for exposures and to have them stop in the middle of an exposure would ruin the job.

Silver is used in high current contactors for minimum contact resistance and work good but still require periodic cleaning if the contactors are used a lot.

For the best performance, virtually zero maintenance and long life, you should use pure tungsten. Some machines I have are approaching 100 yrs old and the gaps still have their original tungsten faces and show little signs of deterioration and work great!

Thoriated tungsten works well but it will release mildly radioactive gases and it more expensive that tungsten.

The common "modern" copper tubing gaps in todays SSTC's are OK and cheap but if you change them out to a heat sink series tungsten gaps you cannot believe the difference in the coil operation.

By far the best gap is a rotary but it is also the most complicated and costly.

Note too, each coil is unique as to the best gap to make it sing. The one gap may not work well one a coil and another gap will be a dramatic difference!

Frank

At 09:12 PM 2/4/2007 -0700, you wrote:
Original poster: Mike <megavolts61@xxxxxxxxx>

So, you are saying my silver gaps didn't work beautifully, even though they sure appeared to???

Original poster: Frank <fxrays@xxxxxxxxxx>

Hi All,
Silver was used in early spark gaps without much success. The silver
vaporizes and oxidizes rapidly and your gap will fire for a short
time and then start sputtering.
It gives off a beautiful green blue flame tho!
The only way you could use silver with limited success is to water
cool the gap and use low currents. Not suited for SGTC's unfortunately.

Tungsten carbide does not work well either, you gap gets very hot and
does not spark well.

Pure tungsten or thoriated tungsten works the best.
Some folks use TIG welding electrodes with good success, it just
depends on the type of compound they used to make the electrode.

Frank


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