In experimental physics, an instrument specially designed for accurately measuring very low temperatures is called what?

Introduction / Context:
This question comes from the topic of measuring instruments in physics. Different physical quantities such as length, temperature, pressure and electric current require specialised devices for accurate measurement. Ordinary thermometers are suitable for everyday temperatures, but extremely low temperatures, for example in cryogenics experiments, demand special instruments. Knowing the name of the device used for low temperature measurement is useful in general knowledge as well as in basic physics.

Given Data / Assumptions:

• The physical quantity of interest is temperature, specifically very low temperatures.
• The options list several unusual instrument names.
• We assume low temperature means values far below the freezing point of water, such as those near liquid nitrogen or liquid helium temperatures.
• We focus only on the standard terminology used in textbooks.

Concept / Approach:
The correct term for an instrument used to measure very low temperatures is cryometer. The prefix cryo comes from a Greek word meaning cold or frost and is used in words like cryogenics, cryosurgery and cryostat. A cryometer is designed to remain accurate and responsive at low temperatures where ordinary mercury or alcohol thermometers may not perform well. The other names in the options either do not correspond to standard instruments or are used in different contexts. Therefore, cryometer is the correct choice.

Step-by-Step Solution:
Step 1: Recognise that instruments for temperature measurement generally include thermometer, pyrometer and cryometer, each for different ranges. Step 2: Recall that pyrometers are used for very high temperatures, while cryometers are designed for very low temperatures. Step 3: Note that the prefix cryo indicates cold, which fits the context of low temperature measurements. Step 4: Examine the options and see that cryometer matches this description directly. Step 5: Verify that the other given names do not represent standard thermometric devices for low temperature measurement. Step 6: Conclude that cryometer is the correct name for an instrument measuring very low temperatures.

Verification / Alternative check:
In discussions of temperature ranges, textbooks often mention ordinary thermometers for everyday ranges, clinical thermometers for body temperature, pyrometers for furnace temperatures and cryometers for cryogenic temperatures. For example, temperatures near the boiling point of liquid nitrogen (around 77 K) or liquid helium (around 4 K) are measured using cryogenic techniques and instruments. The consistent use of the term cryometer or related cryogenic sensors in this context supports the choice of cryometer as the correct option.

Why Other Options Are Wrong:
Diagometer is not a standard term associated with low temperature measurement in physics. Chromatoptometer suggests an instrument related to colour or optics, not to temperature, and is not commonly used in basic physics. Cymometer, if encountered, would more likely relate to measuring wave frequencies (cycles), not temperature.

Common Pitfalls:
Because several of the options sound unfamiliar, students may try to guess based purely on spelling. A helpful strategy is to pay attention to word roots: cryo for cold, pyro for heat and thermo for temperature in general. Recognising these roots allows you to decode many scientific terms and choose the correct option even if you have not memorised all instrument names directly.

Final Answer:
An instrument designed for accurately measuring very low temperatures is called a Cryometer.