I've been wracking my brain for the term for this all week. I'm almost certain there is a single word for the quantity that you make in an experiment, where you can't (easily) measure what you really want.
For example:
When measuring the specific heat of an object using a calorimeter, we are taking temperature data, instead of directly measuring the specific heat. The temperature is called the .....
Not "indicator", "stand-in". Might begin with "pseudo", but don't remember.
Thanks in advance!
Consider calling ‘temperature’ a proxy or measure of ‘heat’.
It is a measure in the sense that there is a relationship between temperature and heat. It is a proxy in the sense that you have something standing in place of the real thing.
measure noun 3.1 An indication of the degree, extent, or quality of something. ‘his resignation is a measure of how angry he is’ - lexico
(NB: the noun’s definition 1 is also relevant, but applies more to a unit such as Celcius as a measure of a quantity such as temperature.)
proxy noun A figure that can be used to represent the value of something in a calculation. ‘the use of a US wealth measure as a proxy for the true worldwide measure’ - lexico
A companion answer to try to clear up some terminology.
Specific heat is a derived quantity.
What Are Derived Quantities? Derived quantities are quantities that are calculated from two or more measurements. Derived quantities cannot be measured directly. They can only be computed. Many derived quantities are calculated in physical science. Three examples are area, volume, and density.
In order to calculate a derived quantity, you must have at least two measurements. Each formal measurement system has a fixed (and rather arbitrary) base unit set. In SI, the base unit set contains seven units - meter, second, kilogram, mole, amp, candela, and degree kelvin. Additional measurements may be required in some particular circumstances such as radian and steridian measurements. But those do not appear as units in the final expression of the derived quantity.
So for specific heat, which is the amount of heat needed to raise one kilogram of something by one degree kelvin, we have units of joules per kilogram per kelvin in SI. Joule is also a derived quantity. In base SI units, it is a kilogram meter squared per second squared. You need measurements of mass in kilograms, temperatures in kelvin, and time in seconds.
One way to calculate specific heat is to measure the mass of a sample and put it in a box with a calibrated resistor wire (premeasured). You measure the starting temp and note the time, switch the current on, measure the current in amps, and the time duration in seconds that you applied the current. Then you measure the final temperature. When combined with the calibrated measurements inside the machine, you have what you need to calculate the specific heat of the material.
So in some way shape or form you must obtain measurements for all the base unit quantities that comprise the derived unit. Often, we can use other derived units to get there quicker. Resistance in ohms is a derived unit, but is usually a known quantity at the outset or measured with an ohmmeter.
Answer to question -
There is no term for what it is you need to measure because that is determined by the device you are using. Machines simplify the process of calculating derived quantities because they already have some of the measurement information needed inside of them. This is what calibration is about.
This is why the term proxy often works when the machine gives you all but one of the measurements needed. You don't need to know anything about the machine, just that it exists and reduces the problem to one measurement. In contrast, if you do know how the machine works, parameter may be a better term, since the machine is no longer a black box, but represents a known algorithm.
a person or thing that takes the function of another (M-W)
In this case I think you'd be using it along the lines of 'as we are unable to easily measure X we are instead substituting the measurement of Y, which is comparable (or can be used to reach the same measurement)'
In a mathematical or logical expression, a symbol that may be replaced by the name of any element of a set. (TFD)
In this case it would be along the lines of 'as we cannot easily measure X, we will be measuring Y as a placeholder and using it to determine the value of X'
a device (such as the pointer on a scale or the gnomon of a sundial) that serves to indicate a value or quantity
something (such as a physical feature or a mode of expression) that leads one to a particular fact or conclusion : indication (M-W)
'The temperature is the index of [whatever you're seeking to measure]'
