## Tuesday, January 13, 2015

### Measurement of Length-Screw Gauge

How do you find the gauge of a metal sheet? How do you find the gauge of a wire that is used in electrical appliances? How do you find the diameter of a steel rod that is used in the construction of buildings? Which measuring device is used in the manufacture of some of small components of machine with accurate measurements?

You get answers to the above questions and learn some other concepts:
Our daily life starts with measurements. We purchase milk in liters (volume) and vegetables in kg (mass). We try to reach the school in time. Measurement of physical quantities play an important role in physics. A physical quantity cannot be understood completely, by describing it or by knowing its qualities. It is better to know its 'measurement' along with the above two factors to understand it completely. If there were no measurements in physics, it would not have been developed to this extent. Physics is otherwise known as science of measurements.

Some times very accurate measurements of some physical quantities are essential. For example, very accurate measurements of mass of the ingredients are required in the  preparation of a drug. Very accurate measurements of components of a car are required to assemble it. So very sensitive instruments are required to measure some physical quantities. The sensitivity of an instrument depends on its least count. An instrument is said to be more sensitivity if the least count of the instrument is minimal.

A number of instruments are used to measure length or distance. To measure very small  length like thickness of a thin plate or diameter of a thin wire, an instrument called screw gauge  is used.

• A screw gauge consists of mainly, a screw and a nut. When the head of the screw is rotated in a nut, the screw moves forward or backward depending on the direction of rotation of the head. The distance travelled by a screw is equal to its pitch, if the head of the screw makes one complete rotation. When the head of the screw is rotated by a small fraction of a complete rotation, the screw moves a distance equal to a small fraction of the pitch of the screw.

• Sometimes this distance becomes the smallest distance that can be measured accurately by using a screw and a nut. This principle is used in the construction of a screw gauge.

• A screw gauge consists of two scales. A pitch scale with an index line is used to measure the distance moved by the screw. A head scale is used to know the number of rotations made by the head of the screw or a portion of the complete rotation made by the head of the screw. The head scale is a circular scale with equally spaced divisions on the head of the screw. Observe the line diagram of a screw gauge and its parts.