What Was in that Sensor Lead?

Normally no explosive items to be used in the home appliances. However, as you have discovered, that it was present in it, you can ask the manufacturer for details. Perhaps spark may be a confusion and Sodium metal likely to be present, which can catch fire when it comes in contact with water. Sodium is used as coolent liquid in Fast Nuclear Reactors and is also used in some lamps for yellow light source. When heated, it can be ionized easily. It is likely that the tube was used to heat the Solid Sodium.

Sodium is shiny silvery gray metal.

I'm glad you brought the subject up .It is the wonder of such things that peaks my curiosity as well .I still take things home from the dump just to see how they work .I began that fascination as a kid trying sometimes successfully to fix discarded machines or simply dissasembling them to see how they worked .In a different time we might attribute your spark to a sign or a work of god but today we search for an answer because we have the ability to find one,while in the past we had no option but to write it off as a wonder . .

You most likely cut the capillary tube going to a sodium bulb thermostat. As was previously posted, sodium will ignite with exposure to moisture and can produce a very hot, quick burning flame. You were lucky it was such a small amount.

I would say you hit it exactly. The configuration you mentioned matches what we saw. I think the only reason that I didn't get more on my hand was that the cutter I used crimped the end of the cap-tube. It's probably a good thing that substance is not used anymore if there is no economical fail-safe for it. I would imagine that most people are as unaware of the risks as I was. I thank you for your input. You've been very helpful. You're right, I was lucky it was a small amount. I would hate to think what would have happened if there had been more that the tenth of a gram or so that did manage to leak out

Nak (sodium - potassium)could be the culprit. It was commonly used in sensing bulbs up into the 1970's. It was also used for self powered thermostatic valves. NaK has a high thermal expansion rate. However if the material leaks from the bulb, capillary, or valve bellows and comes into contact with water it will burst into flames. Product liability issues resulted in it being removed from the market.

Rumor has it that the widow of the enonymous founder of a major controls company died when one of that company's NaK valves started a fire in her bedroom of the family mansion.

Na-K combination is never used as it is a highly explosive mixture. Also K does not yield any light so there is no reason to put into device. In slightly hot condition Sodium (Na) reacts with air and burns easily. It is likely that some heat has been used before cutting the tube. This means the Sodium was in either liquid form or partially in vapor form. This will catch fire quickly and does not require water and perhaps will show a flash. Reaction with water also has a flash with white fumes coming out of it that are Sodium Hydroxide, which is also harmful and can give tears. Sodium fires are dangerous, particularly in heated form. Reaction itself is exothermic and generates heat and helps firther to cause damage. However fire can not spread inside the tube due to no contact with water or O2. This stops the fire. If all Sodium was out and then there was heat or a drop of water then it might have been an explosion.

I have worked in Fast Breeder Nuclear Reactor as a research scientist/Engineer for nearly 22 years and we are worried about Sodium fires as we use tons of Metal Sodium in it as coolent in lots of pipes and in tanks of the nuclear reactor cavity. We also have to exchange heat of hot liquid Sodium with cold water to make steam so there are areas where Sodium pipes pass through water based heat exchanger. If there is some Sodium leak, then there will be explosion. Our people keep close watch on each Atom of Sodium leaking out in air or water. However, they can never be 100% sure that it will not have an eccident. Professional hazards we have to live with. However, at home, I am not sure, if we can love such fires. Play safe while you have fun with junk you never know about fully. I will not say you stop doing all that stuff. This question was very valuable. I greatly appreciate it. I think you can provide link to some pictures of the device that was experimented.

No kidding, you were this close to become a paranormal case.