Did you ever wonder how the 100 light series strings stay lit when a bulb fails?

[LCHIEN]

Christmas lights. Here is a microscope picture of a 2.5 V .43W miniature incandescent light the kind that comes 100 in a series string. (actually its two strings of 50 in series, wrapped together.)





You can see the tiny filament supported by two conductive support posts. You can also see the tiny wire wrapped around the base of the posts. This is the bypass element that keeps the whole string of lights from going out when one filament burns out. The bypass is made of an oxidized aluminum wire. the wire is fairly resistive with a thin coat of oxide and 2.5 volts does not pass much current through it. When the filament fails the entire 120 V is across the bypass and now it punches thru the resistive oxide coating and shorts out the two posts bypassing the open filament. And that's how the string does not go out when one bulb burns out.

The 120 V is divided equally among the remaining bulbs. When too many are bypassed, the 120 volts is divided between less than 50 bulbs, which increases the voltage past the rated 2.5 and they start to burn out fast instead of one here and one there. Eventually too many are bypassed and the string has a short life left. So when you see a few bulbs out, its a good time to replace them. I have a cannabalized string I use for spare bulbs.

[radhak]

Nice write up, Loring! I did wonder about this but never reached out to read up on it.
And I did suspect that after a few bulb failures the stress on the rest must increase.

Btw, a few decades ago I used to work with bulbs that would not allow this fail-safe; so each time a bulb failed the entire series would go dark, and we'd need to go bulb to bulb checking if power reached it, and if not, surmising that the prior bulb had a problem.

[harley00]

I've now got some photos of the microscope and bulb needed - one point is that the microscopes (there are at least two) have been used on a privately funded marine research station for at least 25 years and are still going strong (?!), the other is that mains voltage is 220~240V. The microscopes appear to simply have CH on them and the bulb states 220V 20W SB. Any suggestions for sourcing these at reasonable cost would be much appreciated as they are needed for future work.

[LCHIEN]

A bit off the original topic but I'll answer.

Olympus Optical is still in business. THey sold off the Camera business last year but the medical instrument and microscope business is still going strong.
You might try contacting them and given them the device model and location the bulb is from and see if they have or can tell you the source of the bulbs.

Just looking at the bulb its hard to tell but it looks like it has a concave reflector on one side of the bulb, making it some kind of projector or illuminator maybe for a slide stage. You might take it to a store like 1000 Bulbs and see it they can cross it\

Your picture lacks a lot of useful info ... no sense of scale so it can't be estimated the size of the bayonet socket it fits, A mm measurement of the base dia is useful
Also bulbs are described by the diameter and length of the glass envelope.
It clearly has more numbers on the other sides of the base but you don't show them. Numbers are there for a reason.
The function of the bulb rather than us guessing is also a key piece of info.