NDA (I) 2018 General Ability Test · Question 83 of 150

Correct — B, Barometer. The barometer is the instrument for atmospheric pressure, and the name says so — 'baros' is Greek for weight, and what the instrument weighs is the column of air above it. Torricelli's original design, from 1643, is a glass tube closed at one end, filled with mercury and inverted into a mercury trough; the atmosphere pressing on the open surface holds the column up, and at sea level it settles at about 76 cm. The portable aneroid version replaces the mercury with a sealed metal box that flexes as the air pressure changes, moving a needle over a dial. Either way, the reading is a direct measure of the pressure of the atmosphere, which is why a falling barometer warns of a storm and a rising one promises fair weather.

• (a)Ammeter — An ammeter measures electric current in a circuit, in amperes, and is connected in series so that the whole current passes through it. It has nothing to do with pressure.
• (c)Potentiometer — A potentiometer compares or measures potential difference. Its virtue is that at balance it draws no current from the source, so it gives the true electromotive force of a cell rather than its terminal voltage — an electrical instrument, not a meteorological one.
• (d)Lactometer — A lactometer measures the relative density of milk and is used to detect watering or the removal of cream. It is a hydrometer adapted for one liquid, and although it does involve a fluid, it says nothing about the pressure of the air.

Atmospheric pressure at sea level is about 101 kilopascals, equivalent to a mercury column of roughly 760 millimetres or about 1,013 millibars. Because air thins with height, the reading falls as one climbs, which is why the same instrument, calibrated in metres instead of millibars, becomes an altimeter in an aircraft. Meteorologists in India report the quantity in millibars or hectopascals, and it is the pattern of these readings, drawn as isobars on a chart, that reveals the depressions and anticyclones behind the day's weather.

Instrument questions are pure recall, so the efficient move is to learn them in a single table rather than one at a time, and to notice that the ending '-meter' is doing no work in telling them apart. Worth holding together are barometer for atmospheric pressure, thermometer for temperature, hygrometer for humidity, anemometer for wind speed, hydrometer for relative density of liquids, lactometer for milk in particular, ammeter for current, voltmeter for potential difference, galvanometer for the presence of small currents, seismograph for earthquakes, and altimeter for height. NDA has drawn from this table repeatedly, including a 2023 item that asked only for the UNIT of air pressure rather than the instrument, so both halves are worth knowing.

• A barometer measures atmospheric pressure; Torricelli built the first mercury barometer in 1643.
• At sea level the atmosphere supports a mercury column of about 76 cm, roughly 1,013 millibars.
• The aneroid barometer uses a flexing evacuated metal box instead of mercury, and is portable.
• A barometer graduated in height rather than pressure serves as an altimeter.
• A sudden fall in the barometer reading signals an approaching storm; a steady rise indicates fair weather.

• Reading the '-meter' ending as a clue; ammeter, lactometer and potentiometer all share it and measure quite different things.
• Confusing a hydrometer with a hygrometer — one measures the relative density of a liquid, the other the humidity of air.
• Assuming a barometer reading rises with altitude; it falls, because there is less air above.

NDA asks instrument-to-quantity in both directions, and occasionally slips in the unit instead of the instrument.

• practice — not a real PYQ

  The instrument used to measure the relative humidity of air is the

  • (a)hydrometer
  • (b)hygrometer
  • (c)anemometer
  • (d)barometer

  Answer(b) hygrometer — the hydrometer measures the relative density of liquids, and the anemometer measures wind speed.

• practice — not a real PYQ

  At sea level, normal atmospheric pressure supports a mercury column of about

  • (a)13.6 cm
  • (b)76 cm
  • (c)100 cm
  • (d)1,013 cm

  Answer(b) 76 cm — about 1,013 millibars; 13.6 is the relative density of mercury and 1,013 the value in millibars, both planted as near misses.