I'd welcome an off-the-list discussion. > From: John Woodgate > Sent: Thursday, August 23, 2007 11:34 AM > > Tarver, Peter writes: > > > For accurate information on the underlying principles of how > >thermocouples work, perform searches on "Seebeck Effect." > > There's a long tale about this, which is OT, so I'll > drastically précis. > Two physicists are trying to find out whether the voltage is > in fact generated at the junction. One shortens his wires > progressively and finds that the voltage goes down, so the > voltage must be generated in the wires. But the second argues > that the short wires are hotter at the ends away from the > junction, so that is why the voltage is less. He keeps those > ends at a constant temperature as he reduces their lengths, > even though when they get to 100 microns long, he needs > liquid helium for coolant. The voltage stays constant.
At 100 µm, the technique must have cost a small fortune to implement. Ahhh! The heady sensation of a research grant! > So does that mean that the voltage IS generated in the > junction? Or is it doe to the temperature GRADIENTS in the wires? If T1 and T2 are maintained at constant temperatures and the voltage difference between the wires at their 'free' ends is the same, regardless of length, that provides no true guidance to the truth and the experiment was doomed to fail from the start. The Seebeck Effect predicts that a potential difference is created in the metal, if the two ends are held at different temperatures, without regard to length. If we make the length of the metal arbitrarily short, the voltage will appear to have been created in the junction, but that doesn't tell us anything, because there is still a length of conductor with each end at fixed, but different temperatures. As I understand it, Seebeck noticed this phenomenon in single pieces of metal bar. This adds credibility to the proposition that the potential difference is created in the metal and not at the junction. I think this is correct. The following is a simplistic explanation of how I understand the physics of the Seebeck Effect. In a metallic atom, freeing an electron from the valence band requires a certain amount of energy (the work function). For the same metallic species, the energy to free an electron at one temperature is not the same as it is at another temperature. By holding a conductor's ends at two different temperatures, a difference in work function between ends leads to a potential difference, measurable in units of Joules per Coulomb. If two dissimilar metals traverse the same path through space, thereby having the same temperature at each end (and not having different temperatures at any point along their lengths), two different potential differences are established (one for each metal), the combination of which provide a more readily measurable voltage. In a thermocouple, the only reason to have the ends joined is to establish a common "point" at the measurand and to establish an electrical "common." If you were to simply twist the wires together, you would still be able to measure a voltage at the free end, though it is significantly more susceptible to the effects of vibration and differing coefficients of expansion, making it an unreliable junction forming method. Regards, Peter L. Tarver, PE [email protected] CONFIDENTIALITY This e-mail message and any attachments thereto, is intended only for use by the addressee(s) named herein and may contain legally privileged and/or confidential information. If you are not the intended recipient of this e-mail message, you are hereby notified that any dissemination, distribution or copying of this e-mail message, and any attachments thereto, is strictly prohibited. If you have received this e-mail message in error, please immediately notify the sender and permanently delete the original and any copies of this email and any prints thereof. ABSENT AN EXPRESS STATEMENT TO THE CONTRARY HEREINABOVE, THIS E-MAIL IS NOT INTENDED AS A SUBSTITUTE FOR A WRITING. Notwithstanding the Uniform Electronic Transactions Act or the applicability of any other law of similar substance and effect, absent an express statement to the contrary hereinabove, this e-mail message its contents, and any attachments hereto are not intended to represent an offer or acceptance to enter into a contract and are not otherwise intended to bind the sender, Sanmina-SCI Corporation (or any of its subsidiaries), or any other person or entity. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
