[hamdani yusuf (OP)]

No. They both show the temperature of the contents of the thermometer bulb. One is heated entirely by conduction from the air, one has an additional dynamo component.

How can we measure the temperature of the air around the spinning magnet?

[hamdani yusuf (OP)]

which is why experimental physics is nontrivial - particularly when dealing with heat.

I don't think I have said otherwise.

[hamdani yusuf (OP)]

NASA Probe Touches the Sun! Why Is It Not Melted

Never before has a man-made object come so close to the Sun! The Parker Solar Probe will provide unique measurement data about our star - but for all the excitement about this project, people are moved by one question: why doesn't the probe melt when it's near the Sun?

Using a wrong definition might bring us undesired results.

[alancalverd]

How can we measure the temperature of the air around the spinning magnet?

A thermistor or thermocouple at the focus of a concave mirror, is a very simple thermal telescope. Or you could calculate the heat dissipated into the ambient air from the drag on the spinning magnet, and model the subsequent convection. or use schlieren technique to visualise it. 

[Bored chemist]

No. They both show the temperature of the contents of the thermometer bulb. One is heated entirely by conduction from the air, one has an additional dynamo component.

How can we measure the temperature of the air around the spinning magnet?

Are you asking an actual question, or do you plan to go down a pointless rabbit hole about the impossibility of measuring a system without perturbing it?

[alancalverd]

It's arguable that the mirror-thermistor trick offers a nondisturbing measurement.

A thermistor dissipates heat when you measure its resistance, so if you allow it to heat up gradually you will  (in theory) find an inflection point in the resistance/time curve where its temperature is actually in equilibrium with the target and no heat is being exchanged.

This proposal combines a theory of brilliant simplicity with an experiment of impossible difficulty, but it might amuse a philosopher.

[hamdani yusuf (OP)]

How can we measure the temperature of the air around the spinning magnet?

A thermistor or thermocouple at the focus of a concave mirror, is a very simple thermal telescope. Or you could calculate the heat dissipated into the ambient air from the drag on the spinning magnet, and model the subsequent convection. or use schlieren technique to visualise it. 

This technique filters out any frequency outside the reflective capacity of the mirror. It also filters out any frequency outside the absorption capacity of  thermistor or thermocouple.
It's also more likely to capture the radiation from behind the air, rather than representing the temperature of the air itself.

[alancalverd]

Whish is why we use a focussed array. If you want to be really clever, substitute a microwave antenna and use the analysis of Penzias and Wilson to estimate the temperature of your target, then heat up the antenna.

What level of accuracy do you want? Why? (Experimental physicist talking here)

[hamdani yusuf (OP)]

Whish is why we use a focussed array. If you want to be really clever, substitute a microwave antenna and use the analysis of Penzias and Wilson to estimate the temperature of your target, then heat up the antenna.

What level of accuracy do you want? Why? (Experimental physicist talking here)

What's the range of temperature it can measure?
Typical accuracy in industrial instruments is 0.5% of measuring range. It's practical for the system to be controlled within operating condition.

[alancalverd]

Thermistors work from around -50C to 300C. I have used them to measure temperature changes of 1mK to +/- 1μK. If you want a traceable calibration in the room temperature range, no real problem - use an oil bath and a platinum resistor to fix absolute points to better than 0.05K

[hamdani yusuf (OP)]

Thermistors work from around -50C to 300C. I have used them to measure temperature changes of 1mK to +/- 1μK. If you want a traceable calibration in the room temperature range, no real problem - use an oil bath and a platinum resistor to fix absolute points to better than 0.05K

You will need reflector in infrared range instead of microwave.

[Bored chemist]

Thermistors work from around -50C to 300C. I have used them to measure temperature changes of 1mK to +/- 1μK. If you want a traceable calibration in the room temperature range, no real problem - use an oil bath and a platinum resistor to fix absolute points to better than 0.05K

You will need reflector in infrared range instead of microwave.

So, for example, aluminium foil instead of aluminium foil.

[hamdani yusuf (OP)]

So, for example, aluminium foil instead of aluminium foil.

I was comparing with common microwave antenna, which Alan mentioned.

[alancalverd]

You could use a thermistor, but a tuneable antenna would probably be easier if you want to analyse the microwave spectrum. Your result would presume that the target volume was effectively a black body but I guess the behavior of air below 2.4 GHz is a good approximation. I'm not sure what final antenna Penzias and Wilson were using to determine the cosmic background: AFAIK the receiver was an exponential horn waveguide.

[hamdani yusuf (OP)]

I'm not sure what final antenna Penzias and Wilson were using to determine the cosmic background: AFAIK the receiver was an exponential horn waveguide.

I found a Naked Science video about this.

Big Bang Discovery - Penzias & Wilson

380,000 years after the Big Bang the Universe had expanded to the size of the Milky Way.  It had cooled from billions of degrees Fahrenheit to a few thousand.  As it cooled the electrons slowed down.  The Universe was now ready to make its first true elements.

One of the scientists who discovered this critical moment in the story of the Universe was Arno Allan Penzias.  In 1963, a 30 year old Penzias and his 27 year old colleague Robert Woodrow Wilson began work on the Holmdel Horn Antenna in New Jersey.  Initially they were only studying cosmic radio waves, but they would stumble upon one of the greatest discoveries of all time.

Clip taken from our documentary ?Birth of the Universe?.

[alancalverd]

And the temperature of fresh pigeon poo is about 313K.

[Bored chemist]

Your result would presume that the target volume was effectively a black body but I guess the behavior of air below 2.4 GHz is a good approximation.

Better guesses are available.
In particular, better guesses should be available to anyone whose field of interest includes radar which depends on microwaves travelling through air for long distances.

[alancalverd]

Long-range radar generally works in the kHz to MHz range where water vapor absorption is minimal.

[Bored chemist]

Long-range radar generally works in the kHz to MHz range where water vapor absorption is minimal.

In clear air, what's the typical range of microwave radar?

[Bored chemist]

No. They both show the temperature of the contents of the thermometer bulb. One is heated entirely by conduction from the air, one has an additional dynamo component.

How can we measure the temperature of the air around the spinning magnet?

We get a bunch of thermometers.
It would be a dumb choice but, just to make the point, we can use a mercury thermometer, an alcohol thermometer a gallium thermometer, a water thermometer  and a hexane thermometer.
And then we measure the electrical conductivity of the filling liquids.
We can plot the apparent temperature against the electrical conductivity of the filling liquid and then we can extrapolate to zero conductivity.

Science has been doing that sort of thing for centuries.
As I have said before, you should try learning it.