2. Instruments for Physical and Mechanical Quantities

This is the largest and most frequently tested category in SSC/Railway exams. It covers instruments that measure pressure, temperature, humidity, wind speed, mechanical force, density, and ground vibrations.

Many of the instruments in this section are also the backbone of everyday weather forecasting. A meteorological observatory typically runs a barometer (or barograph) for pressure, a thermometer for temperature, a hygrometer for humidity, an anemometer for wind speed, and a rain gauge for rainfall, side by side, and it is the combined trend across all of these readings — rather than any single instrument alone — that allows meteorologists to forecast weather. Exam questions sometimes test whether a candidate can correctly match each weather element (pressure, temperature, humidity, wind) to its correct instrument, so the pairing given below should be learnt precisely rather than approximately.

2.1 Barometer

Barometer. An instrument used to measure atmospheric (air) pressure. The mercury barometer, invented by the Italian scientist Evangelista Torricelli in 1643, consists of a glass tube closed at one end, filled with mercury, and inverted into a mercury reservoir; the height of the mercury column supported by atmospheric pressure (normally about 76 cm at sea level) indicates the pressure. A falling barometer reading generally indicates an approaching storm or bad weather, while a rising reading indicates fair weather, which is why barometers are extensively used in weather forecasting (meteorology). The aneroid barometer is a mercury-free variant using a flexible metal capsule that expands or contracts with pressure changes, making it portable and the basis of the altimeter.

2.2 Thermometer and Pyrometer

Thermometer. An instrument used to measure temperature. The common clinical/laboratory mercury (or alcohol) thermometer works on the principle that liquids expand and contract uniformly with changes in temperature; the liquid rises or falls in a thin, calibrated capillary tube. Daniel Gabriel Fahrenheit developed the mercury thermometer and the Fahrenheit temperature scale in the early 18th century, while Anders Celsius introduced the Celsius (centigrade) scale. Digital thermometers use electronic temperature sensors (thermistors) instead of a liquid column.

Pyrometer. An instrument used to measure very high temperatures — such as those inside furnaces, kilns, or molten metal — that are far beyond the range of ordinary liquid-in-glass thermometers. Pyrometers work on the principle of measuring the intensity of radiation (infrared or visible light) emitted by a hot body, since radiated energy increases with temperature; this makes them non-contact instruments, useful for measuring temperatures where direct contact is impossible or unsafe.

2.3 Hygrometer

Hygrometer. An instrument used to measure the humidity of air, i.e., the amount of water vapour present in the atmosphere. A common type, the wet-and-dry bulb hygrometer, uses two thermometers — one with a dry bulb and one with a bulb wrapped in a wet wick; the difference in their readings (due to evaporative cooling of the wet bulb, which depends on how much moisture the air can still absorb) is used to calculate relative humidity. Hygrometers are important tools in meteorology, agriculture, and industries such as textiles and pharmaceuticals where humidity control is critical.

Relative humidity, the quantity most hygrometers report, is expressed as a percentage — the ratio of the actual amount of water vapour present in the air to the maximum amount the air could hold at that temperature before becoming saturated. This is also why the wet bulb reads a lower temperature than the dry bulb whenever the air is not already saturated: evaporation of water from the wet wick absorbs heat from the bulb, and the drier the surrounding air, the faster this evaporation and cooling occurs.

2.4 Anemometer

Anemometer. An instrument used to measure the speed and, in some designs, the direction of wind. The most common type is the cup anemometer, invented by John Thomas Romney Robinson in 1846, which has three or four hemispherical cups mounted on horizontal arms that rotate when struck by wind; the speed of rotation is proportional to wind speed and is converted into a reading. Anemometers are essential instruments at meteorological stations and airports.

2.5 Speedometer and Tachometer

Speedometer. An instrument fitted in vehicles to measure and display their instantaneous speed. Traditional (mechanical) speedometers work using a flexible drive cable connected to the gearbox, which rotates a magnet inside a metal cup (the speed cup); the magnetic field induces eddy currents that create a drag proportional to speed, deflecting a needle against a calibrated spring. Modern vehicles mostly use electronic speedometers based on sensor signals.

Tachometer. An instrument that measures the rotational speed of a shaft or engine, expressed in revolutions per minute (RPM). It is commonly seen on vehicle dashboards (helping drivers avoid over-revving the engine) and is also used in industrial machinery to monitor motor speed.

2.6 Spring Balance

Spring Balance. A simple instrument used to measure the weight (force due to gravity) of an object. It works on Hooke's Law, which states that the extension produced in a spring is directly proportional to the force (load) applied to it, within the elastic limit. A pointer attached to the spring moves along a calibrated scale as the spring stretches under the weight of the suspended object.

2.7 Hydrometer

Hydrometer. An instrument used to measure the relative density (specific gravity) of liquids, such as milk (lactometer, a specialised hydrometer), battery acid, or alcohol. It works on the principle of flotation (Archimedes' principle): a sealed glass tube weighted at the bottom floats vertically in the liquid, and the depth to which it sinks depends inversely on the liquid's density — it floats higher in denser liquids and sinks lower in less dense liquids. The reading is taken at the liquid surface against a calibrated stem.

2.8 Manometer

Manometer. An instrument used to measure the pressure of a gas or fluid enclosed in a vessel or pipe (i.e., gauge pressure), most simply as the difference between two pressures. A simple U-tube manometer consists of a U-shaped tube containing a liquid (such as mercury or water); the difference in the heights of the liquid columns in the two limbs of the tube, when one end is connected to the gas/system under test, gives a direct measure of the pressure difference.

2.9 Viscometer

Viscometer. An instrument used to measure the viscosity of a fluid, i.e., its internal resistance to flow (a measure of 'thickness' or friction between fluid layers). Common designs measure either the time taken for a fluid to flow through a narrow capillary tube under gravity, or the force needed to move an object (such as a falling ball) through the fluid, and relate this to viscosity using standard formulae. Viscometers are widely used in the petroleum, food, and paint industries to check the quality and consistency of oils and other liquids.

Viscosity generally decreases as temperature increases for most liquids (which is why cold engine oil pours far more sluggishly than warm engine oil), so accurate viscosity measurements are always taken and reported at a specified, controlled temperature to make results comparable between different tests and different fluids.

2.10 Seismograph / Seismometer

Seismograph (Seismometer). An instrument used to detect and record the intensity, direction, and duration of earthquakes (seismic waves) travelling through the Earth. It typically works on the principle of inertia: a heavy mass suspended from a support tends to remain stationary due to inertia while the ground and the instrument's frame vibrate during an earthquake; this relative motion between the stationary mass and the moving frame is recorded (traditionally on a rotating drum with a pen, now electronically) as a seismogram. Earthquake magnitude is commonly reported on the Richter scale, developed by Charles Richter, while the older Mercalli scale measures earthquake intensity based on observed damage and human perception rather than the energy released.

2.11 Calorimeter

Calorimeter. An instrument used to measure the quantity of heat involved in a physical or chemical process, such as heat gained or lost during a change of state, a chemical reaction, or the combustion of a fuel. The commonly used water-equivalent (mixture) calorimeter works on the principle of conservation of energy — heat lost by a hot body equals heat gained by a cold body (and the calorimeter itself) when they are mixed in an insulated container, allowing the specific heat capacity or calorific value of a substance to be calculated from the observed temperature change.

2.12 Dynamometer

Dynamometer. An instrument used to measure mechanical force, torque, or the power output of an engine or motor. It typically works by applying a controlled, measurable resisting load (braking force) to the rotating shaft of the engine under test and measuring the force needed to hold that load steady, from which torque and power can be calculated. Dynamometers are widely used in automobile testing and engineering laboratories to rate engine performance.

2.13 Chronometer

Chronometer. A highly accurate timekeeping instrument, historically developed for marine navigation, where precise knowledge of time is essential to calculate longitude at sea. The marine chronometer was perfected by the English clockmaker John Harrison in the 18th century, whose spring-and-balance-wheel mechanism kept accurate time despite the motion of a ship, unlike pendulum clocks, which are disturbed by rocking. The word is now used generally for any timepiece of very high precision.

2.14 Rain Gauge

Rain Gauge. An instrument used to measure the amount of rainfall (precipitation) that falls at a given location over a given period of time, usually expressed in millimetres. A simple rain gauge consists of a wide, calibrated funnel that collects rainwater into a narrow measuring cylinder, so that even a small amount of actual rainfall produces a proportionally larger, more easily readable rise of water in the cylinder. It is one of the most basic and widely used instruments at meteorological observatories.

2.15 Bolometer

Bolometer. A highly sensitive instrument used to measure the intensity of radiant heat (infrared radiation), including extremely faint radiation such as that reaching Earth from distant stars. It works by detecting the minute rise in electrical resistance of a fine, blackened metal strip as the strip absorbs incoming radiant energy and warms up very slightly; this change in resistance is measured using a sensitive bridge circuit and converted into a measure of radiation intensity. It was invented by the American astronomer Samuel Pierpont Langley.

2.16 Barograph

Barograph. An instrument that continuously records atmospheric pressure over time, producing a graphical trace on a rotating drum. It is essentially a self-recording aneroid barometer: a stack of aneroid capsules expands and contracts with pressure changes and drives a pen arm that traces the pressure variation onto a paper chart, which is useful for meteorologists tracking pressure trends (rather than a single reading) to forecast weather.

Quick Revision Table — Physical/Mechanical Instruments