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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Excellent stuff TDU! Can we see a picture of an air streamer?
BTW, I'm sure it says in the Colorado Springs Notes "Must examine sparks with rotating mirror" but Tesla never got round to it, and neither did anyone else that I know of
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
The streak photography article is fascinating. It is also about 3 orders of magnitude faster than what I am doing and requires a photomultiplier tube to amplify it. It gives propagation rates of leaders of 10^9 cm/sec (approx 1/30 of speed of light) whereas I can only achieve 10^4 cm/sec. Still, I was never expecting to be able to see things like that with equipment found around the home. On the other hand, streamer growth has structure on very slow timescales which is why they are interesting to look at. In short, you can see them move so there are things happening at all sorts of timeframes from nanoseconds to seconds. Streamer brightness is much lower however but should register some interesting images. Interpretation of streak camera stuff is easy if sparks are a straight line but become difficult if angled or branched so a blurred mess is a possible outcome when I try this with streamers. I'm not sure how "useful" this will be but I hope to get some streamer data sometime.
The photo (one of my "mystery" photos) shows that the smear of light in the path of the spark is an artifact since it disappears when the spark is out of the top of the screen. I am not sure if this is due to mirror edge effects, or is a lens flare or camera UV issue. The green colour is a result of auto exposure and probably auto white balance when the software processes purple sparks.
There is another photo from the book here that shows continuous and flash mode streak pics:
Super fancy streak cameras can do <2pS and some laboratory thngs are into the femto seconds!! I am not sure much "moves" in that case
Pie-in-the-sky stuff for "us", but maybe we can learn some tricks from them.
Before Tesla was born, John Tyndall was doing the expiriment described here with moving a mirror or turning of one's head quickly.
I have a 1860's physics book (lost at the moment) that shows how to use a funnel with a paper diaphram with a bunsen burner to modulate the flame and use a spinning mirror to "see" the sound waves... I am sure Tesla was familiar with this but I am not sure here ever tried it. He probably though the events were too fast to see. Or, he did try it and could not get it to work given the time and equipment he had.
The leaders in the book are made with a Marx generator I think so they are different than ours. Tesla coil leaders might be much slower since they ride the ~100kHz sine wave of the secondary as opposed to being a giant cap discarge thing. We can look and see now
The "smear of light" you mention is where the camera is looking down the leader from the end thus it appears to have a bright area in it. People commonly mistake these for ball lighting in standard arc photos.
You picture seems completely true to me. You can probably "fix" the white ballance on your camera which might help the colors if you don't already have enough camera settings to worry about
The bending of the arcs just makes the pictures more "interesting" since we can argue for years as to how that affects the pictures
Great stuff!!
Probably be a whole week or two before I can get all the parts for mine and get it going
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Terry has a nice setup (see the next post). It should have a good chance of working well and it has a lot more design about it than my unbalanced shaft and small mirror. Having the camera nice and close is a big plus and the lexan shield is close enough to the lens not to worry about internal reflections. I'm not sure the ISO is really increased by 4 times since you are just increasing your chance of catching a single event in the mirror by 4 times.
A few more pics for comment
Photo 1 shows that sometimes the spark has no following ringdown sparks.
Photo 2 shows some mild ringdown only.
Photo 3 shows the bright white arc channel interrupted with faint purple arcs.
Photo 4 shows detail of the initial spark which has a well defined bright central channel on the enlarged view.
Photo 5 shows the ionization around the stainless steel electrode which does glow red hot at the end of a run although that is too faint to see.
Photo 6 shows that the ionization is sometimes delayed by 5 us after the initial spark strikes.
Photo7 shows an unusual streak that I suspect is the spark channel hitting a dust mote and burning it up.
I got the motor and mirrors ordered for my streak camera. I also made the frame, mounted the parts, and machined an aluminum block to hole the mirrors. Here is the camera system so far:
Just simple wood construction. The bottom has a tripod 1/4-20 adaptor for my heavy tripod. The big pillow block will hold the motor. The aluminum cube will have the mirrors expoxied to it and then it will be pressed on the motor shaft. The lexan shield protects the camera in case it blows apart. There is a 1/4-20 brass bolt on the bottom of the camera and spacer with a wing nut to hold the camera to the base. Took about two hours to build up.
Here is a picture (850k) through the camera as mounted:
So just need to drill the block for the motor shaft and press it on and glue the mirrors in place.
It should do 0.5in/uS at 20,000 RPM at ten feet. So a 100kHz coil should have the + - cycles separated buy 2.5 inches at full speed. With four mirrors and having the camera so close it should have a wide field of view so I don't have to take 10,000 pictures
The camera has manual zoom, focus and ISO (100,200,400) settings and has 2048 x 1536 pixels. At 24000 RPM that is 400 RPS or four frames so the ISO really should be 1600. But I think a friends camera will do that if it turns out to be a problem. The mount is easy to adjust to any camera with a drill and saw
Should be cool!!
For Peter's pictures:
Photo 1 shows that sometimes the spark has no following ringdown sparks.
I wonder if the hot streamer arc channel was alread in place. Or it might have just gone in one shot like a Marx spark. It does seem more streaked at the right if that is were the spark began.
Photo 2 shows some mild ringdown only.
I would think the dim part would be the ring "up" and then the arc would drain the system and be dark immediately after that.
Photo 3 shows the bright white arc channel interrupted with faint purple arcs.
I have no idea!! Maybe the dim parts happen to be hotter and more conductive.
Photo 5 shows the ionization around the stainless steel electrode
Interesting that the ionization is fairly constant as one would expect
Super cool! Prolly far more questions than answers
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Yes. The bright spark at the top is the initial one, then the next 5 or so progressively weaker sparks below are subsequent events so down is advancing time.
With this photo I have increased the contrast to show two things. Firstly that there are two types of electrode glow on the right with the greenish one appearing only after a few cycles. Secondly, look closely at the ringdown sparks at the left. The space between the sparks is not empty but shows another line in between. I wonder if this is a second harmonic.
Yes. The bright spark at the top is the initial one, then the next 5 or so progressively weaker sparks below are subsequent events so down is advancing time.
Ok... This is what I thought was going on which is apparently wrong:
This is a pretty typical top voltage of a coil when an arc reaches ground:
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Here is the "first" streamer picture which forms an arc that hits.
Pic is of about 12 inches of an 18 inch spark from the toroid side on the left. I was throttling the variac back to try to just get streamers and few hits.
It is quite different. Time axis is downward. The initial streamer sparks (the top one) can be broken into perhaps 6 consecutive channels (5us apart = 2 pulses per 100kHz). Although it is difficult to be sure, only the last one makes it across the screen then a 10us gap then the main arc hits. Interestingly there is no ring down on the main arc, however the distances are greater and intensity is down.
I've taken lots of black shots with information on them but need to process them.
I guess the new information from the rotating mirror stuff is that streamers enlarge with successive cycles and ring up leading to a spark that connects. Sparks that connect (often) have a ring down. Not really unexpected from the CRO pics but nice to see it directly. So streamers ring up and sparks ring down - easy to remember.
First pic shows the streamer with ring up then an arc connects. The yellow blur on the right is my kitchen window seen through the mirror when it is horizontal and mostly out of view. Second pic is lots of overlapping sparks just for fun. Conventional sparks to a grounded object like I have shown before with the ringdown visible. The sparks are somewhat out of focus at the bottom of the pic.
Here is the "first" streamer picture which forms an arc that hits.
What a stunning picture!!!!!!!!!
I enhanced and commented it here:
You can see every known leader phenomena in it!!!!
Here is a reference from Bazelyan/Raizer.
It looks like the leader tip stalls a bit probably as that cycle reverses and cuts off the power. The next cycle reheats it and eventually it goes onward and breifly stalls again.
Amazingly, you can see the streamer branches(!!!) at the very tip on the right where it is darker.
Interestingly there is no ring down on the main arc
That is a hit or miss thing. Often the power is totally removed from the streamer so the arc is dead right there. Models predict that based on stuff that still needs to be studied. It is probably fairly consistant (if it will occur or not) with a given coil setup.
I've taken lots of black shots with information on them but need to process them.
Looking forward to more!!!
This is fantastic stuff!!! Your pictures are some of the best available of any leader arc and probably far better!!!
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High speed Tesla spark photos
I am not sure much "moves" in that case 
