Page 31 - Physics - XI

Sources of Error

1. The screw may not have uniform threading, therefore, pitch may not be constant at diff erent places of
the screw.
2. The screw may have friction.
3. Circular scale divisions may not be of equal size.
4. The sheet may not be of uniform thickness.
5. The spherometer may have backlash error.

VIVA VOCE

Q1. Why is determination of zero error not required in case of a spherometer?
Ans. In case of a spherometer, we always take the diff erence between fi nal and initial readings. Hence,
zero error, if any, gets automatically eliminated. That is why, determination of zero error is not
required.
Q2. Give reason for the three legs of a spherometer.

Ans. Because at least three points are required to represent a plane and also for the stable equilibrium,
three is the minimum number.
Q3. What is the position of the screw with respect to the three legs?
Ans. The tip of the screw is equidistant from the three vertices of the equilateral triangle formed by the
three legs.

Q4. What do you mean by radius of curvature of a spherical surface?
Ans. Radius of curvature of a spherical surface is the radius of the sphere whose spherical surface forms
a part.

Q5. State the principle of the spherometer.
Ans. It works on the principle of micrometer screw. Spherometer is used to measure the thickness of the
small thin plates and radii of curvature of spherical surfaces.

Q6. Why is it called a spherometer?
Ans. It is used to measure radii of curvature of spherical surfaces [sphere + meter, i.e. to measure radius
of sphere].

Q7. Defi ne the term pitch of a spherometer.
Ans. Pitch is the distance between two consecutive threads of the screw taken parallel to the axis of rotation.
It is also defi ned as the distance moved by the screw in one complete rotation of the circular scale.
Q8. How can the accuracy of a spherometer be increased?
Ans. We know that an instrument is more accurate with the smaller least count. The least count can be
decreased by decreasing the pitch and increasing the number of divisions on the circular scale.

Q9. Which instrument will you prefer to measure the thickness—a spherometer or a screw gauge
with the same least count?
Ans. We prefer a spherometer as in case of a spherometer there is no zero error or zero correction to be
taken. It is because we take the diff erence of initial and fi nal readings.

Q10. What is the radius of curvature of a plane surface?
Ans. Infi nity
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