Platinum Resistance Thermometer

Some years ago, when I taught Physics A level in schools, I had a very bright student (Tom went on to Oxford).  One of the practicals was called the ‘Platinum Resistance Thermometer’.   It required students to change the temperature of a metal coil and measure the corresponding relative resistance at that temperature to determine the temperature coefficient of resistance of the wire.  Ostensibly the purpose of the experiment was to verify whether the wire was made of platinum or not (it was a private school, but even so the budget did not extend to having several thousand pounds tied up in a thick strand of platinum in the physics department).  Though mostly submerged in oil, the coil of thick wire was easily visible inside the boiling tube, and its ends, poking through the rubber stopper and necessary for completing an electric circuit, were obviously copper.

Tom worked diligently over three hours, patiently ensuring that the oil in the tube in a water bath was at steady state before measuring the temperature and it’s corresponding resistance compared to a length of Constantan wire forming the metre bridge.  He checked the potential difference across the wire was the same on each reading, which he repeated a fixed number of times at each temperature to get an average, and he even ensured that the ambient temperature in the vicinity of his experiment was the same for all measurements.  His data was excellent, and I expected his write-up would be top draw, showing clearly that the wire was  – wait for it, wait for it – copper.

Imagine my surprise when I came to mark his work and saw his conclusion that the wire was platinum.

Tom had carefully massaged each and every result and calculation, rounding up or down just a little as needed, factoring in errors in a biased way, ‘adjusting’ the line of best bit, and so forth, until he got a result that was very nearly the same as the standard value for platinum.  When I re-worked his calculation using the data he’d obtained I got very nearly the value of copper!  To get the result Tom got he’d had to work backwards from the established value for platinum…it must have taken him hours and hours to do.  If he’d just have trusted his data and worked from it he’d have produced a perfect piece of work.  Instead he was going to have to throw it away, a waste of his time.

This is a true story, and it’s an important lesson.  I frequently receive applications and enquiries from people asking if they are suitability for a particular job we are advertising at Tutors International.  They look at whether they want to do the job rather than what the job description states.  Having convinced themselves that they are the ideal person for the role they then try to explain how their skills and experience show that they’re perfect — even when completely unsuitable. You’d not believe how many applications I’ve received from non-Americans for a job that specifies an American is required.

Tom learned his lesson when he was at school…how come so many teachers have not yet learned it?

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