How can I measure the internal temperature of a solid metal? I’m heating up several metal blocks as part of an experiment to determine their specific heat capacities. In a nutshell, I’m heating the blocks of known mass to 100 °C, then adding them to an insulated vessel containing water of known temperature and measuring the increase. Pretty basic stuff.

I’m trying to work out how I can measure the temperature of the blocks, so that I know they’re at 100 °C. Leaving them in the boiling water for a long period of time is not ideal – I want to be sure that they are at the desired temperature that I can record.

How can I do this effectively? I thought about using an infrared thermometer, but wouldn’t that just measure the temperature on the surface of the block?

I also thought about something to do with measuring the resistance of the metals, but is this actually accurate and easy to do? Any advice or suggestions are appreciated.
You can use ultrasound thermometry. It uses change in speed of sound under heat. AFAIK, there are industrial-grade solutions on the market.