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If you have the rod already, you could try measuring its electrical resistance and see if it falls in to the range for steel, or is significantly higher as alloys with chrome/nickel tend to be (such as stainless).

If you have a 5A power supply, or a car battery and a headlamp bulb (along with safe means of holding it as it will get HOT!) you can use that to put a decent current along the rod. Then measure the volt drop from two points on the rod (not on the power supply leads or clips) and from a measurement of the cross-sectional area and length you ought to find the material's resistivity. You will need something that can measure a few mV probably, but a cheap-ish multimeter ought to do.

Carbon steel is about 1.43E-7 Ohm.meter, while stainless about 6.9E-7 Ohm.meter, Other cases can be found here:

EDIT: Just to add some background info about the Kelvin (4 wire) resistance measurement method:
 
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Some pendulum clocks were extraordinarily accurate, such as these:
Hi pc1966. I'll address your following post shortly.
The clock stuff: All practiising electrician will know that their craft basically begins with Michael Faraday (Michael Faraday - Engineering and Technology History Wiki - https://ethw.org/Michael_Faraday) - nearly 200 years ago.
Soon after (1841) Alexander Bain patented the first electric clock (Distant Writing - Bain - https://distantwriting.co.uk/bain.html)

A free swinginging pendulum devoid of ANY interference WHATSOEVER will keep good time. In my clock, with the advantage of 21st century electrics, were to be encased in a chamber with a fixed low vacuum it would probably equal the performance of the Shortt/Synchronome. "Interference" in mechanical pendulum clocks, may come from poking the pendulum on every swing and extracting energy to move the hands.

In 1842 Mathtias Hipp came along with a better version (A DIY Free Pendulum, Hipp Toggle master clock. - http://www.rogerj.co.uk/clock.htm)
In 1895 Frank Hope-Jones formed the Synchronome company (Synchronome Clocks history - https://www.synchronomeclocks.com/pages/about-us) and his master clocks, with one second pendulums were in use well into the 1960's - when they finally succumbed to the Quartz crystal and other esoteric resonating sources. Along the way..in 1921, collaboration between Hope-Jones and William Hamilton Shortt produced the time reference you linked too.. Now we all know !!
 
If you have the rod already, you could try measuring its electrical resistance and see if it falls in to the range for steel, or is significantly higher as alloys with chrome/nickel tend to be (such as stainless).

If you have a 5A power supply, or a car battery and a headlamp bulb (along with safe means of holding it as it will get HOT!) you can use that to put a decent current along the rod. Then measure the volt drop from two points on the rod (not on the power supply leads or clips) and from a measurement of the cross-sectional area and length you ought to find the material's resistivity. You will need something that can measure a few mV probably, but a cheap-ish multimeter ought to do.

Carbon steel is about 1.43E-7 Ohm.meter, while stainless about 6.9E-7 Ohm.meter, Other cases can be found here:

EDIT: Just to add some background info about the Kelvin (4 wire) resistance measurement method:
I have the rod and may attempt to try it. It has also occured to me that if I supported it off the side of a stout board, fixed one end and with a dial gauge at the other..... I could pour boiling water over the rod and see if it expands...
Thank you..I'll follow up the links.. Roger
 
Roger
Sorry to be a spoilsport - coming back to your original question (Invar or steel), it occurs to me, and no doubt has to you too, that a simple experiment might be worth trying (but only if the length of your thermostat rod will fit in your oven!)
Cut a wire cotahanger to be as close as practicable to the length of the rod, and bung them in the oven at as high a temp as you like! Take them out, stand the pair next to each other and see if they're the same length!!
Or maybe there's a flaw in this scheme 🤔
 
Roger
Sorry to be a spoilsport - coming back to your original question (Invar or steel), it occurs to me, and no doubt has to you too, that a simple experiment might be worth trying (but only if the length of your thermostat rod will fit in your oven!)
Cut a wire cotahanger to be as close as practicable to the length of the rod, and bung them in the oven at as high a temp as you like! Take them out, stand the pair next to each other and see if they're the same length!!
Or maybe there's a flaw in this scheme 🤔
Done ! I left the bitter end of the brass tube on (as a socket) and cut a length of steel fence wire to fit under the conical under face of the "nut" on the other end of the stat rod. the other end was trapped under the nut and screwed down so they lay alogside each other. With other half safely watching Paddington Bear I set the oven to 180 and after 10 minutes put the combination in. The fence wire bowed away from the specimen by 1/4"..soon cooled down to as was. A strong indication therefore that the stat rod is Invar :)
Simplest ideas !!!!
 
Is it possibe that the thermostat is actually a pressure switch with some sort of thermal fluid in the sensing rod, cut a bit off the end??.
 
Where I find this stat interesting is where a single stat is used in a top mounted immersion where normally sink & bath lives are switched manually depending on the volume of HW required, one terminal on each heating element are permanently linked to each other and the thermostat switches the common neutral.
the thermostat rod length is matched to the longest (bath) heating element, on a standard HW cylinder with 11" and 23" elements the stat length will be 22/23".

I wonder what effect just fitting a 11" stat would have. Sink selected no problem. Bath selected and assuming a cold cylinder then the temperature will rise uniformly until 60C is reached and the stat switches Off, if you then start using HW the cold water will displace the HW until the contents of the cylinder are almost exhausted, temp falls to (by say 5C, hysteresis) to 55C, stat switches ON, cylinder reheats. The only? advantage I see with using a 22" stat is that it may switch on once a smaller vol of water has been used, some of the stat will be at 60C and some at say 15C (mains temp) so perhaps this is enough to switch on the stat.
 
Just to clear up the above point, all top mounted twin immersions with a common stat means that the stat pocket & stat will never be longer than the short element, both elements will work fine, and the stat will cut out at its setpoint temperature. If the stat was the same length as the longer element, and if this (bath) element is selected it will cut out at its setpoint temperature and cut in and start reheating the cylinder once the incoming cold water is in contact with a fairly small length of the stat.
If the shorter (sink) element is selected then the stat just might cut out (if at all) but at a very high temperature as "1/2" the stat will be ~ at mains water temperature and the other "1/2" might be at 100C if a vented cylinder.

I did a few simple tests on a new 11" stat that I found lying around, the stat, when fully immersed in water switched out at 62C and switched in at 50C, with 4.5" immersed, it cut out at 73C and switched in at 67C. Stat nominally set to 60C for both tests.
 
All very interesting but..... The point of the thread was to discover if anyone knew for certain that the thin steel rod inside the stat's brass tube is Invar.. I think we're 90% certain it is.. That's all..
 
Must admit I don't know exactly how it all works, the willis immersion heater is clearly different from direct, and going in from side of tank is also different from going in from top, but it seems unlikely that all thermostats work in the same way.
 

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