Scientific Instruments — Set 6

Correct Answer: B. Compass

• **Compass** = A magnetic compass uses a magnetised needle that aligns itself with Earth's magnetic field, with the north-seeking pole pointing toward magnetic north. • **magnetic declination** — The compass needle points to magnetic north, not true geographic north; the difference (magnetic declination) varies by location and must be corrected for accurate navigation. • 💡 Wrong-option analysis: Gyroscope: determines orientation by angular momentum, not magnetic field; Sextant: measures angles between celestial bodies and the horizon; Radar: detects objects using radio waves.

Correct Answer: A. Potentiometer

• **Potentiometer** = A potentiometer measures electromotive force (EMF) accurately by balancing the unknown EMF against a known potential difference from a standard cell. • **null deflection method** — At balance, no current flows from the source being measured, making it more accurate than a voltmeter; also used as a variable resistor in circuits. • 💡 Wrong-option analysis: Ohmmeter: measures resistance; Wattmeter: measures electrical power; Ammeter: measures electric current.

Correct Answer: A. Telescope

• **Telescope** = An astronomical telescope collects and focuses light from distant celestial objects (stars, planets, galaxies) to form a magnified image. • **Galileo's discoveries** — Galileo used a refracting telescope in 1609 to discover Jupiter's four largest moons (Galilean moons), revolutionising astronomy. • 💡 Wrong-option analysis: Periscope: views objects above obstacles using mirrors, not celestial bodies; Spectroscope: analyses light spectra to determine composition of stars; Microscope: magnifies tiny nearby objects, not distant ones.

Correct Answer: A. Speedometer

• **Speedometer** = A speedometer displays the instantaneous speed of a vehicle, typically in km/h or mph, and is mandatory in all motor vehicles. • **electronic sensors** — Modern speedometers use electronic sensors on the transmission or wheel hub; it shows current speed, unlike the odometer which shows total distance. • 💡 Wrong-option analysis: Chronometer: measures precise time; Tachometer: measures engine RPM (shaft rotation speed), not vehicle speed; Odometer: measures cumulative distance travelled, not instantaneous speed.

Correct Answer: C. Measuring heat radiation

• **Measuring heat radiation** = A bolometer measures the power of incident electromagnetic radiation (especially infrared) by detecting the temperature rise it causes in an absorbing element. • **infrared astronomy** — Bolometers are extremely sensitive and are used in infrared astronomy to detect heat from distant stars and galaxies that are too cool to emit visible light. • 💡 Wrong-option analysis: Measuring wind speed: done by an anemometer; Measuring sound intensity: done by an audiometer or sound level meter; Measuring atmospheric pressure: done by a barometer.

Correct Answer: C. Sphygmomanometer

• **Sphygmomanometer** = A sphygmomanometer is the standard medical instrument for measuring arterial blood pressure using an inflatable cuff and a pressure gauge. • **systolic/diastolic** — Blood pressure is expressed as systolic/diastolic (e.g., 120/80 mmHg); used with a stethoscope to detect Korotkoff sounds during deflation. • 💡 Wrong-option analysis: Audiometer: evaluates hearing sensitivity; Stethoscope: listens to heart and lung sounds but cannot measure pressure numerically; Pyrometer: measures very high surface temperatures.

Correct Answer: A. Clinical Thermometer

• **Clinical Thermometer** = A clinical thermometer is calibrated specifically to measure human body temperature, ranging from 35°C to 42°C (95°F to 107.6°F). • **constriction in stem** — It has a constriction near the bulb to prevent mercury from falling back immediately, ensuring the reading holds until reset; normal body temperature is 37°C. • 💡 Wrong-option analysis: Hydrometer: measures liquid density; Barometer: measures atmospheric pressure; Pyrometer: measures very high temperatures (600°C+) remotely.

Correct Answer: B. Photometer

• **Photometer** = A photometer measures light intensity (luminous flux per unit area) and is used in photography, spectroscopy, and environmental monitoring. • **lux** — Light intensity is measured in lux (lm/m²); a photometer compares the intensity of a light source against a calibrated standard. • 💡 Wrong-option analysis: Rain Gauge: measures rainfall in millimetres; Salinometer: measures salt concentration in water; Lactometer: measures specific gravity of milk.

Correct Answer: B. Sonar

• **Sonar** = SONAR (Sound Navigation And Ranging) uses sound wave propagation underwater to detect objects, map the seafloor, and measure ocean depth. • **1,500 m/s in water** — Sound travels ~1,500 m/s in seawater (4.4× faster than in air); SONAR is the primary tool for submarine detection, wreck finding, and fish locating. • 💡 Wrong-option analysis: Radar: uses radio waves (not sound) and works in air, not underwater; Dictaphone: records spoken words for transcription; Lidar: uses laser pulses in air, not sound underwater.

Correct Answer: C. Hygrometer

• **Hygrometer** = A hygrometer measures atmospheric moisture (relative humidity); high humidity affects weather patterns, human comfort, and preservation of materials. • **relative humidity** — Relative humidity of 100% means air is fully saturated; libraries and museums use hygrometers to protect books and artefacts from moisture damage. • 💡 Wrong-option analysis: Thermometer: measures temperature, not moisture; Barometer: measures atmospheric pressure; Hydrometer: measures density of liquids, not atmospheric moisture.