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EXPT NO: 03. SPHEROMETER

Aim : To determine the radius of curvature of a given spherical surface by a

Spherometer.  

Apparatus : Spherometer, plane glass plate, watch glass, scale. Etc.

Diagram:

Fig SPHEROMETER

Fig 02 Fig 03

Formula : Radius of curvature of a spherical surface,

where              d = mean distance between two legs of a Spherometer        

h - sagitta of the spherical surface.

Procedure :         

      Place a plane paper on this table and gently press the Spherometer on it so that the pointed ends of the three legs of the Spherometer produce impressions on the paper. Measure the distances ,  between the three marks of the legs. Hence find the mean distance 'd" between the legs.

      Find the least count of the Spherometer.

      Place the Spherometer on the plane glass plate. Rotate the screw till the tip of the screw just touches the glass surface. This can be ensured by observing that the tip of the screw coincides with its image in the glass plate.

      Note down the main scale reading A by observing the position of the edge of the circular scale on the main scale. Note circular scale reading (B) by observing the circular division coinciding with the edge of the main scale.

      Find the total reading.

       In this way, take three observations and calculate its mean.

      Place the Spherometer on the concave surface.

      Raise the screw through some distance. Place the Spherometer on a concave  surface of the  watch glass. Rotate the screw so that its tip and all legs of the Spherometer touch the concave surface. Again note down the main scale reading and circular scale reading, Find the total reading.

                  Take three observations and calculate its mean.

                  Place the Spherometer on the convex surface.

                  Rotate the screw through some distance. Place the Spherometer on a convex surface of the  watch glass. Rotate the screw so that its tip and all legs of the Spherometer touch the convex surface. Again note down the main scale reading and circular scale reading,

                  Find the total reading.

                  Take three observations and calculate its mean.

 Observations :

Distance between the legs ,,

ii)         Smallest division on main scale =………………….cm

 iii)    Distance through which the screw advances on the main scale in 10 rotations of the circular scale  =………………….cm.

iv)                    Pitch of the screw,

v)                              Total number of divisions on the circular scale, N =………………….cm

vi)                             Least count (L.C.) of the Spherometer =   Observation Table :

Object	Obs No	Main Scale reading A cm	Circular Scale reading B	C=BxL.C	Total reading T=A+Ccm	Mean reading cm
Plane Glass Surface	01
	02
	03
Concave Surface	01
	02
	03
Concvex Surface	01
	02
	03

Calculations :                                                                                                                    

Sagitta for the concave surface =

Sagitta for the convex surface,=

Calculations :

Result:                                                                                                                               

Radius of Curvature  for  concave surface =…………..cm

Radius of Curvature  for  convex surface =…………..cm

Precautions :

      Measure distance ‘d’ accurately.

      Consider lowest mark on the main scale as zero mark.

      Rotate the screw gradually so that tip of the screw just touches the centre of the curved surface.

      Add suitable weights in the hanger so that the system is in equilibrium and the common.

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