Sunday, August 21, 2011

Room temperature diamagnetism with pyrolitic graphite

Updated 2011.09.01

I just acquired some pyrolitic graphite and powerful neodymium magnets from United Nuclear. Pyrolytic graphite has the highest diamagnetism of any room temperature material [see correction below]:
Diamagnetism is the property of an object which causes it to create a magnetic field in opposition to an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the magnetic dipole moment. According to Lenz's law, these electrons will oppose the magnetic field changes provided by the applied field, preventing them from building up. The result is that lines of magnetic flux curve away from the material.
Here are some small pieces of pyrolitic graphite on top of four 0.5" square neodymium magnets:

Click to enlarge
This is not magnetic repulsion. If that were the case the pieces of graphite would simply fly off the magnets. The magnetic field is strongest at the edges of the magnets so the largest piece is trapped in the center; it's pushed inward to the point of least magnetism. If perturbed it will snap back to this same point and orientation. The smaller pieces are standing on edge because they are repelling the magnetic field on both sides and can't fall over. If one is pushed over it will immediately snap back into a vertical position.

In this view you can clearly see the largest piece levitating and the smaller pieces standing on edge. The medium size piece is tilted slightly. This may be because its thickness is slightly uneven, which would make the repulsion slightly stronger on the more massive side. This calls for more experimentation.

Click to enlarge
Needless to say this behavior is completely unintuitive and surprising. When pushed the pieces react in a totally unexpected way. It's as if they've fallen into a sort of bizarre magnetic well. Here's a video showing this behavior in real time.

Superconductors have an even more powerful diamagnetic effect but require liquid nitrogen to cool them sufficiently.

I'll have more on this topic in the future as I've also acquired some bismuth, which has the highest room temperature diamagnetism of any metal.

Correction (2011.08.31)

Here's the difference between bismuth and pyrolitic graphite (without the math):

"The most strongly diamagnetic material is bismuth, although pyrolytic carbon may have a [lower] susceptibility in one plane."

So there you have it. I played around with some bismuth the other day and it was quite interesting. Stay "tuned"...

Friday, August 19, 2011

Crookes tube first test

My Crookes tubes showed up today and I had time to get a couple of quick shots of magnetic deflection in action with my invisible magnet:


Thursday, August 18, 2011

Induction coil—another new toy

I have a bunch of HV stuff coming in this week, starting off with this induction coil:

Click to enlarge
I got it from I'm going to be powering Crookes tubes with it so I didn't pay the extra money for something bigger. Once you get over 15-20kV X-rays become a hazard. I have enough problems already. Like the fact that my Crookes tube order is sitting in a facility in Michigan and not moving. So much for Priority Mail.

Wednesday, August 17, 2011

Ball gap for the Dirod electrostatic generator

I finally have the ball gap to show off. It's not finished—the capacitors (Leyden jars in the old days) are rather crummy and will probably have to be redone but I managed to get some action shots anyway.

Construction is simple, just follow the instructions at the end of the manual. I had to guess at the tubing lengths but this seems about right. The most time-consuming part was cutting the bases off the dummy doorknobs. Sadly, my camera was occupied with time-lapse experiments and I didn't get any pictures of the process. It took a couple of hours, a moto-tool with an emery wheel, face mask and goggles. The trick is to cut the metal in narrow strips, working them down until they're small enough that they can be broken off with pliers. I slowly worked my way around until the knob came loose.

After grinding down the stumps of metal stick out of the knobs I glued them to pipe caps with metal-filled epoxy. They don't look as snazzy as I'd hoped but they do work.

With the capacitors I get a very loud snap with a .75" gap. It's the middle of summer right now; next winter will be the real test.

Monday, August 8, 2011

Laser power testing


Several months ago I set out to measure the powers of all my lasers. What seemed like a simple process became a protracted hassle. As with my Dirod electrostatic generator I made a number of mistakes. First the DVM module that I had on hand wouldn't work so I had to buy another one. Next I discovered that the power sensor and DVM grounds have to be separate so I had to come up with another power supply for the DVM. Then I discovered that the IR output from the DPSS lasers was so high that I needed a filter in order to measure the visible output alone. Finally, I checked my meter's sensitivity using the supplied test circuit. That last thing didn't get done because I was futzing around with everything else. When I finally got around to it I discovered that I needed to use the power sensor's built-in shunt resistor to get accurate readings.

At this point I'm visiting this topic for the third time now. The second post was never finished and the first one has been deleted because all the readings I got were completely wrong. Let's start all over from the beginning...

Test Rig

The first thing I had to do was build a test rig. Here's the cradle before its paint job:

Out the anode
The next thing I acquired was a 200mW Calibrated Laser Power Meter Sensor from Lasersbee. You simply attach it to a Digital Voltage Meter and the reading in millivolts is the power of the laser. The power sensor is calibrated for 650nm lasers. For other wavelengths a conversion chart is provided. To make things look cool I bought a back-lit DVM module from Jameco. I found that the sensor and the DVM module need to have separate power supplies to work properly together. Right now I'm using batteries. For the sensor I got a lithium 9V from the local Batteries Plus store. It should last forever as far as this project is concerned. The DVM module is running off of AA batteries.

The sensor itself appears to be some sort of photovoltaic device. It's relatively small so it can be tricky getting the entire beam aimed at it. Unlike gas tube lasers, diode lasers have an output beam that's rectangular rather than circular. The beam has to be collimated to bring it to a point. My 405nm Blu-ray diode laser is so poorly collimated that it isn't possible to get the entire beam on the sensor even with it rotated 45° (making it go corner-to-corner)

Laser Pointers

Laser pointers are a very popular item these days. They're cheap and the green ones are very bright to the eye. I got mine from DealExtreme, one of those "buy direct from China and wait four weeks for it to arrive" places that have sprung up over recent years. Despite the delay everything I've bought from them has been a really good deal. Note that they do offer a lot of junk so caveat emptor. (I don't think I'll be buying any of their condoms.) There are also tons of laser pointers for sale on ebay but it's a crap shoot. Most of the sellers say "5mW" and then lead you to believe it's actually a lot higher. They often take a 5mW and crank the power up. It works—for a while. Take a look at their feedback before you buy. When it comes to diode lasers you typically get what you pay for.

In some countries you can't import lasers at all so check before ordering. Here in the US you can't import laser pointers >5mW so don't bother ordering anything rated higher. You can, however, buy domestically manufactured "laser parts" of virtually any power. See Mad Science For Sale for dealers.

Having said all that, we're about to see that the "≤5mW" rule isn't always followed...

Power Measurements

I used a fog machine to make the beams visible in the following tests. The density of the fog varies so the brightness of the beam doesn't correspond to the actual power. Also, these are 5 second exposures so the digits on the meter change and blur the readout sometimes.

First up is the 405nm pointer with a Blu-ray diode. Note that 405nm is near-ultraviolet so you really don't want to be staring into this one. Here's the 405nm in the test cradle:

405nm ~24mW
This works out to about 25.5mW give or take the error range. The correction factor for 405nm is so high that the error range is quite large, ± a couple of milliwatts. As it happens, a friend of mine tested this laser with his pro-quality meter (with a pyrometer) and got exactly 24mW so I'd say we're doing pretty well there considering the limited accuracy.  And 24mW is a bit higher than 5mW so I definitely need my googles on when working with this one. Shame on Dealextreme for offering 5mW and delivering 24mW.

DPSS Lasers

Next up are the diode-pumped solid-state or DPSS lasers. They consist of two IR lasers and a frequency-doubling crystal. An 808nm laser pumps a 1064nm laser and its output has its wavelength halved to 532nm. The major hazard of DPSS lasers is that their efficiency is less than 100% so there's plenty of invisible infrared light being emitted. At 5mW or less this is not considered a hazard, but at higher powers it is. Remember, you can't see it so you don't know how bright it is or where it is. Because of this, DPSS lasers over 5mW should have an IR filter on the output. None of these cheap pointers of Asian origin have any discernible IR filtering. See this video for a demonstration of the problem. Note that you can also find videos that show how to disassemble a pointer and increase the power, burn things, etc. You're on your own with that stuff.

In order to measure both the visible and IR outputs of my DPSS lasers I had to acquire a good IR filter. In a wild stroke of luck I found a good one on-line for $6. It has some impressive specs:
  • Wavelength—Transmission—Color
  • 470.0nm—97.14% (blue)
  • 532.0nm—97.15% (green)
  • 808.0nm—0.09% (IR/near-IR)
  • 1064.0nm—0.53% (IR)
  • Material: B270
  • Glass in accordance with MIL-G-174
  • Surface quality: F/F per MIL-C-48497A
  • AR coated both sides
  • Filter Requirements: Angle Of Incidence: 0 +/-5 degrees
  • In band transmission: >95% @520~ 550 nm
This filter is perfect. It passes 97% of the green and practically none of the IR. True, I had some trouble finding a way to mount it with the angle of incidence to within ±5°. But I'm going for ballpark measurements here.


It finally occurred to me that any assumptions about the absolute power of the IR emissions from these lasers are unfounded. The relative levels of the 808nm and 1064nm lasers are unknown to me. The Laserbee sensor has two different correction factors for the two IR wavelengths—one greater than one, the other less than one. If the ratio of the two were constant then the calculation could be made.

When I find out more I'll update this article again.

More Power Measurements

Up first is a pointer of Asian origin that I traded a 405nm for. The picture is a bit dim as I forgot to turn all the lights on at this point and the fog is rather thick:

532nm #1
Unfortunately, I didn't manage to get a decent picture with the IR filter in place but the reading was 1.1mV, which about 1.8mW. With a reading of 4.1mV with all three wavelengths combined, obviously there's a lot of IR emission here and the visible power is well below 5mW.

Next up is a Dealextreme 532nm. It doesn't look as bright because there's less fog here:

532nm #2
16.3mV combined.

532nm #2 + IR filter
About 11.2mV (18.25mW) visible. Lots of IR. Say, wasn't this supposed to be "≤5mW"? Heh.

Up next is my AixiZ 50mW OEM diode head:

532nm #3

532nm #3 with IR filter
About 60.6mW visible. Clearly this laser has some IR filtering but I can't tell how much. 50mW is very bright. I had to wear safety goggles to align the beam.

Fortunately, unlike the visible beam, the IR isn't well collimated because of the large difference in wavelength. It's spread out over a larger area which reduces the danger slightly. In this picture, I have the 50mW head shining through my googles, which have an O.D. of 7+ at 532nm and pass nothing but a faint, tiny yellow dot of light. They attenuate the IR as well by at least 40% but a lot still makes it through. Beyond the goggle lens is an IR detector. As you can see, the dot is quite large:

Detecting IR
Green gas tube laser

While I'm working with green lasers, here's a 543.5nm helium-neon gas tube laser:

543.5nm HeNe
That's about 1.8mW, which is nominal for this tube. Tubes use high voltages and glow like a neon sign screaming "Mad Scientist," which is their whole appeal. The power supply for this one is 2600V at 6.5mA. I know I'll never see another large green tube again (actually, a used head went really cheap on ebay the other day but anyway...). I'm still looking for orange and yellow...

Laser tubes have a very small, round (Gaussian curve) bean with a very low divergence. Here's the entire tube in all its glory:

Out the cathode—Click to enlarge
Now you can see what the rest of the test cradle is for. At 19" it's the longest tube in my collection. A red laser tube this size would probably be 20mW. Green is not very efficient. Note that this tube is emitting from the cathode. I have another green tube in my collection that emits from the anode (it's in the first picture at the beginning of this post). It's 0.1~0.2mW. That's tenths of a milliwatt. The beam is so dim you can stare straight into it—not that I'm recommending that...

Next time around—lasers of the visible red kind.

Wednesday, August 3, 2011

Calculator Heaven

In this age of instant karma and overnight obsolescence it may seem surprising that decent calculators are still being manufactured. Fortunately, that's one thing about HP that hasn't changed in recent years.

Several years ago my beloved HP 20s suddenly died. I turned it off, then back on—nothing happened. Dead. I bought a new calculator at Walmart. It was only $12 for a TI with all the same features but it was the flimsiest piece of crap I've ever attempted to use. The buttons have no tactile feedback and don't even make contact about 10% of the time. I can do the same operation three times and get three different results. I've been feeling calculator buttons in stores looking for something like the good old days but everything feels like mush. At least I haven't been accosted by store security yet for feeling up their merchandise.

After seeing used used calculators going for ridiculous price to HP addicts and collectors I finally broke down and bought the newer HP 35s. The keys aren't quite as stiff as before but they still have that solid feel I've been missing. As a bonus I've completely given up algebraic entry for RPN. I put it off for decades but not any longer. Fortunately I have experience using a stack-oriented programing language (FORTH) so RPN is a snap for me.

It's so nice to know that it got entered right the first time every time.

Monday, August 1, 2011

Lots of detail today

Took one look in the eyepiece today and knew I'd hit the jackpot. Sunspots so big I could use them to focus in live view:

Click to enlarge
Unfortunately I've still got uneven illumination and it appears that the 2X Barlow has introduced Newton's rings into part of the image (move back a bit and look at the upper left quadrant).

Fortunately, the spots came out extremely well:

Click to enlarge
Two steps forward, one step back. Another day with sweat in my eyes.

Yet another danger

Just as I was finishing up a successful round of imaging, this showed up:

Good thing I'm not allergic.