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Why does a joule heater element heat up from the middle outwards to the ends? In the picture below, the kanthal mesh heating element is shown heating up from the middle outwards. I'm looking for an explanation of why that is.

enter image description here

The image is a screen grab from a video on the web.

The specifics of the heating element are not pertinent to the question as the same is true of all joule heating elements. Or at least, appears to be so looking at those in my toaster and an electric fan heater.

Buk
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    This question would be improved if (a) the picture were less blurry, (b) we had some idea of the current flow through the mesh, (c) we had some idea about airflow through the mesh, (d) are the mesh wires constant-thickness, or do they have smaller diameter/higher resistance in the high-temperature region, (e) is the mechanical connection between the mesh and the housing also a good thermal connection. This might (or might not) be an off-topic engineering question — though it's interesting and I look forward to an answer. – rob Sep 25 '21 at 15:06
  • My first guess would be that it's a resonance-like effect from heat increasing the resistivity. The center heats up a little bit more than the edges because of slightly less radiative surface area, gets a little bit more resistance because it's a little bit hotter, heats up a little bit more because it has a little more resistance, and so on until steady state is reached with radiative power dissipation. – g s Sep 25 '21 at 16:15
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    You might find this writeup interesting on why Joule heating of a suspended beam can cause parabolic and catenary-like temperature profiles with the peak at the middle. The same analysis could be extended to a mesh. – Chemomechanics Sep 25 '21 at 16:32
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    @Chemomechanics Thanks. That I think answers my question. In the case of the mesh element in the image, the terminals at either end are relatively high mass brass, and act as heatsinks thus drawing the heat away from either end of the mesh; hence the middle heats up first and then spreads out to either end. – Buk Sep 25 '21 at 19:11

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