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Proving Science
Blog TempMeter : Measuring Temperature with a Brass Rod
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  • Author Author: dubbie
  • Date Created: 7 Aug 2020 1:53 PM Date Created
  • Views 1009 views
  • Likes 9 likes
  • Comments 20 comments
  • temperature measurement
  • brass rod
  • provingsciencech
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TempMeter : Measuring Temperature with a Brass Rod

dubbie
dubbie
7 Aug 2020
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Proving Science

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For the Proving Science Project14 activity I thought I would have a go at measuring temperature using a technique I remember being discussed when I was at school, originated by a fellow pupil rather than the teacher, but not actually seen in action. It was in a physics class looking at the expansion of materials with temperature and it was a mind-changing moment for me as I realised that science and engineering stuff was so exciting and anyone could think about stuff and not just teachers and old people. I like thinking about stuff and often do it in the garden sitting in the sun with my eyes closed.

 

The method works by using the expansion coefficient of a metal. A metal with a good expansion coefficient is brass (11 x  10 −6   F −1   ) and I just happen to have some brass rod (for the Art Project14 competition). Aluminium is better at 13 x 10 −6 F −1   but I do not have Aluminium rod so brass it is. I will be using Fahrenheit rather than Celsius as I have a digital temperature meter and this provides slightly better resolution. I am using a 30 cm rod of 2.0 mm rod as the expansion length which rests on a 1.0 mm brass rod. As the long rod expands and contracts due to temperature the smaller brass rod will act as a roller and rotate forwards and backwards as well. By placing a pointer on the 1.0 mm roller rod I will obtain an indication of temperature.

 

As all (?) materials expand with temperature this means the base will as well so I need a base with the lowest expansion coefficient I can find. Diamond seems to be the lowest but as I do not have a sheet of diamond to hand I have decided to use reconstituted stone from an old fire place. It should be somewhere between brick (about 3) and marble (about 6). Plus, it is very straight and very smooth and shiny so the roller should move freely. I gave the surface a good clean (wiped with a damp cloth - that's clean enough!) and clamped a piece of wood at one end as a stop for the brass rod - using a blob of Blue-Tac to just hold it in place. In order to get some friction on the roller I used a flat steel ruler as a weight place near the roller. This will affect the rolling but it will be of minimal impact - hopefully.

 

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For a brass rod of length L = 300mm having an expansion coefficient of e = 11 x  10 −6   F −1   then a 1 degree Fahrenheit increase ( δ T = 1) in temperature will lead to an increase in the rod length of:

 

  δ L =  L x e  x  δ T

          = 300 x 11 x  10  −6  x 1  mm

          = 3.3 x 10 −3   mm

 

The smaller brass rod used as a roller has a diameter of d =1.0 mm giving it a circumference of:

 

C = d π .

    =   π    mm π

 

The angle turned through due to the expansion is:

 

θ   =  δLdπ x 360  degrees

 

So a 1 degree increase in temperature in Fahrenheit will lead to a change in angle of :

 

θ  = ( 3.3 x 10 −3  x 360) /  π    degrees

     = 0.38 degrees

 

The clock face has 12 hour divisions with 5 minute divisions in each hour totalling 60 divisions for the full clock face. Therefore 1 division represents an angle of 360/60 = 6 degrees. So a 20 increase in temperature will lead to (20 x 0.38 ) 7.6 degree change in angle which is just over one increment on the dial. I would have to say that this is not going to be that useful. Still, the technique does work but might need some refinement.

 

A longer brass rod would help, as would a thinner roller, maybe a needle or a pin. It is a fairly hot day today so if tonight is cooler, maybe a change of 20 F then it may be possible to see the rotation on the dial. If it happens I will amend this blog to include it. Otherwise I will have to wait for a much colder day, which could be several days!

 

Dubbie

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Top Comments

  • dougw
    dougw over 3 years ago +7
    Great demo of thermal expansion. You mentioned the expansion of the table. Would it be more accurate to subtract the table coefficient from the brass coefficient - and using the differential coefficient…
  • DAB
    DAB over 3 years ago +7
    Very good experiment. I ran into a real world issue about thermal expansion back in my younger days working in a laser lab. We had connected a camera and lens assembly with aluminum bars and discovered…
  • genebren
    genebren over 3 years ago +6
    Very cool (or in this case hot)! Very interesting experiment. Funny how some ideas stick in your mind for years and years, only to pop up and be the inspiration for an experiment. Well done!
  • DAB
    DAB over 2 years ago in reply to aspork42

    The really fascinating things happen when you begin to look at the effect at the atomic level.

    When I was putting my last book together I sat down and calculated how many photons needed to be absorbed by an atom using the one calorie experiment.

     

    It was the thermodynamic interactions of atoms that led me to the conclusion that it is photons, not electrons that makes electronics work.

     

    DAB

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  • dubbie
    dubbie over 2 years ago in reply to aspork42

    James,

     

    I have also wondered about thermal expansion for years. The starting point of this approach was in Secondary School in a physics lession. It only took me another45 years or so to get around to doing anything about it.

     

    Dubbie

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  • aspork42
    aspork42 over 2 years ago

    Cool experiment!

    Thermal expansion is one of those things that I’ve often wondered about and not considered “solved”. Things like bridges just are forced to “deal with it” by using expansion joints. But we’ve also learned how to use it to our advantage with things like bi-metallic strips in thermostats...

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  • beacon_dave
    beacon_dave over 3 years ago in reply to beacon_dave

    "...I seem to recall that they used this technique with the early transatlantic cables to amplify the weak signals for telegraphy purposes."

     

    "...Thomson's mirror galvanometer (land type) used at Valentia Island end of the original Atlantic cable..."

    https://collection.sciencemuseumgroup.org.uk/objects/co32961/thomsons-mirror-galvanometer-1858-galvanometer

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  • dubbie
    dubbie over 3 years ago in reply to beacon_dave

    Dave,

    A mirror might be a good idea as this will allow the pointer to be longer which will amplify the small movements more. I'll have to think about how  might attached a small mirror to the rod end.

     

    Dubbie

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