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Common measuring instruments – IV

Kedar Kulkarni

The teacher had asked the students to come prepared with names and uses of more instruments. As soon as she entered class the students greeted her and were ready to take down notes.

She listed all the instruments they had already discussed – ammeter, multimeter, altimeter, thermometer, tachometer, odometer, sphygmomanometer, vernier caliper, micrometer, barometer, anemometer, hydrometer, lactometer and a magnetic compass.

She said the list was elaborate so far but she would still add more instruments. “Does anyone know what a spherometer is and what are its uses?” asked the teacher before answering “It is used to measure the radius of curvature of a sphere. This way we can also measure the radius of curvature of the lenses.”

The students were silent, perhaps because they had never heard of such an instrument before. The teacher realised that and said: “It is used to carry out pulmonary function test which measures the quantity of the air inhaled and exhaled by us. It is very useful in diagnosing some lung diseases like asthma, bronchitis, etc.” The students made a note of these interesting points.

Moving on to the next instrument the teacher asked if anyone knew about an opisometer. The class was stunned. The teacher smiled and said: “It is used to measure the lengths of curved lines. It is basically used to measure the lengths of rivers and roads listed on maps. We know that the maps are drawn to scale. It has geared mechanism to convert the line distance on the map to kilometres or miles. The sophisticated versions used today are called chartometers.”

She continued “The next one is simple, the weighing scale.” The entire class chimed together and said it is used to measure the weight of the items we buy. “Right, we use a weighing scale to measure the weight of a given object. How many types of weighing scales do we commonly see?” she asked.

There is the two pan scale wherein the object is placed in one pan and the weights in the other; and the spring type weighing scale used in laboratories which is called a mechanical weighing scale as no electricity is require to use it.

Electronic weighing scales also use a calibrated strain gauge to measure the weight. The weigh bridges at the octroi centers or in factories can weigh up to 30 tonnes. Whereas the ones used in chemical laboratories or those with goldsmiths weigh very small weights accurately. Weighing scales need to be certified by the government agencies of weights and measures every year.

(Writer is a mechanical engineer and runs a hands-on science activity centre at Margao)

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