Microscopy Image Gallery

Exploration of the Month

December 1997

There is possibly nothing that raises the spirit of the holiday season than lights around a house or on a tree. This month we look more closely at these lights with the help of a magnifier.

Tree lights Tree lights

The pictures above show two different christmas tree light bulbs. One is a regular light bulb, the other is the flasher bulb that is included with most sets of lights which makes the entire string of lights flash on and off when it is used. The major difference between the two images is that the first one is taken with the light reflecting off the bulbs, and the other one has the light shining through from behind the bulbs. In the first image it is much easier to see the reflections of the glass bulb which encloses the light than in the second image. The glass bulb is used to keep air away from the filaments inside. If the light bulb was operated in air it would burn out very quickly indeed and not be very practical at all.

Filament of a tree light Filament of the flashing light

Here we see the insides of the two different bulbs more closely. On the left is the ordinary bulb. It has a coiled Tungsten metal filament which is connected between two metal pins. When we turn on the light bulb electric current flows up one pin, through the coiled filament and down the other pin. The current flowing through the thin Tungsten filament causes it to heat up to a very high temperature. At this temperature the filament is white hot, and shines very brightly, making the whole bulb glow.
On the right is the flasher bulb. It has an extra pin which touches one of the filament support pins below the point where the filament is supported. In this bulb the electric current flows through this extra pin up to the support pin, then through the filament and down the other support film as before.
This extra pin is different from the other two. It is actually made of two different metals joined together in the middle, and is called a bimetallic strip. As the current flows through this pin, it too heats up like the Tungsten filament, but not nearly so much. As it heats up the metal layers expand, like most things do when they are heated up. Now these two metals have been carefully chosen so that they expand by very different amounts when they are heated. The only way for one side of the pin to expand more than the other and still be joined together is for the pin to bend, so as the pin heats up it will also bend. The flashing effect is made by making the bimetallic strip bend away from the filament as it heats up, because as it bends away it breaks contact, and the electric current stops flowing. With no current there is no light, and the light bulbs go out. Since there is no current flowing the bimetallic strip cools down, and bends back until contact is made again. Then the electric current flows, the light comes on, and the bimetallic strip starts to heat up again....

SEM image of light filament Higher magnification Even higher magnification

These three images are taken in the scanning electron microscope, and show increasing magnification pictures of the tungsten coil, showing more detail. In the first image we can see the basic shape of the coil, but not much else. In the second image at 4 times higher magnification, we begin to see structures on the tungsten wire. The third image shows these features more clearly. The grooves running along the length of the wire are formed as the wire is made. The metal is pulled (or drawn) through small holes to get it to the right diameter. small imperfections around the edge of the hole make these scratches in the length of the wire. Also visible in this image are the individual grains of metal that make up the wire.

The images below are also taken in the scanning electron microscope, but this time we are looking at the contact. The first image shows the bimetallic strip. The row of dark dots represents the junction between the two layers. Notice how all the "scratches" in each layer line up with each other, but not with those in the other layer. The second two images show the contact tip of the flasher bulb resting against the filament support. In the low magnification image it looks fairly sharp and clean, but at higher magnification (in the last image) it looks very jagged and rough. This is a result of all those making-and-breaking contacts. Every time that happens there is a little spark that makes a new pit in the surface. Over time the tip gets very rough, as you can see here. Can you think of other things that might look like this?

SEM image of bimetallic strip SEM image of contact area Higher magnification

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Last Update: 12/13/97