Students came prepared to learn about more instruments. Rohan went up to the board and listed out some instruments as anemometer, hydrometer, lactometer and a compass. As the teacher entered class she noticed the board and smiled. Guessing that Rohan had written the list she said: “So we will discuss these instruments today.”
The class began with the anemometer. Teacher asked what it is used to measure.
Meera said: “It is used to measure the speed of the wind. It is a very common device in any weather lab. It uses three or four cups mounted on the horizontal bars which in turn are fixed to a vertical rotating shaft. The wind pushes the cups from the hollow side and creates a torque which then rotates the vertical shaft. The speed of wind is then calculated in proportion to the speed of rotation.”
Teacher then said there are different types of anemometers such as vane anemometer, laser doppler anemometers, ultrasonic anemometers, etc.
Talking about hydrometers Anthony stood up and said it is used to compare the density of liquids to that of water. A lactometer is a type of hydrometer that compares the density of milk.
“Those used for measuring the alcohol content are alcoholmeters,” said the teacher. “The ratio of the density of liquid to that of water is called its specific gravity. Hydrometer is made of a cylindrical glass tube ending in a bulb at the bottom. Some mercury is filled in it to make it float vertically. The liquid for which the specific gravity is to be determined is poured in a long graduated cylindrical container. The hydrometer is then lowered into it. Depending on the density of the liquid the hydrometer sinks to a certain mark. The scale reading on the hydrometer which aligns with the topmost level of the liquid in that cylinder indicates the specific gravity of that liquid. It rises in higher density liquids and sinks in lower density liquids. Lactometer is used to find the specific gravity of the milk.
The class then began to discuss about the compass.
Rahul said: “It is not a measuring instrument but a device which indicates the north-south direction. We all have learnt that when a bar magnet is freely suspended in the air it points towards the earth’s north-south direction. And the side facing the north is considered the magnet’s north pole. But since like poles repel, the geographic north pole of the earth is considered as the magnetic south pole.