The instrument used for measuring the temperature is called a-
A. Voltmeter
B. Thermometer
C. Calorimeter
D. Ammeter

Hint: In this question we will discuss the instrument used to measure the temperature, the principle behind it. Different types of that instrument and how it was made. We will also discuss various scales of temperature measurement and how they originated.

Complete step-by-step answer:
The instrument used to measure temperature is called a thermometer. It is a combination of two words thermo and meter meaning heat and measurement, respectively. The most common form of thermometers used are mercury thermometers. There we have a small container of mercury (bulb), which is brought in contact with the object whose temperature must be measured. Then according to the temperature of the body, the volume of mercury shrinks or expands. Then a predetermined scale on the thermometer can be used to know the value of temperature corresponding to the current volume of mercury. The three most commonly used scales of temperature are the Kelvin scale named after Lord Kelvin, this scale is considered as the absolute scale of temperature. Then we have the Celsius scale, one unit on this scale is the same as the Kelvin scale, however its values are according to the physical properties of water at the zero on this scale water freezes and at 100, the water evaporates. The third scale is the Fahrenheit scale. It was defined as zero for the freezing temperature of brine solution and 96 for the average temperature of the human body, although now it has been redefined with 32 being the freezing temperature of water and 212 being the boiling point of water. It must be noted that all the scales are linear.

So, the correct answer is “Option B”.

Note:
With new technology we now have electronic thermometers, which don’t use mercury and instead use electronic devices to measure the temperature. Their advantage is that they are way more durable and do not rely on a poisonous substance like mercury. Thermometers using Infrared Radiations to measure the temperature of a body have also been developed and are used to get the temperature without any contact.