Measuring Instruments : Spherometer

Spherometer : A Spherometer is an apparatus which is based on the theory of screw . It is usually used for measuring thickness and radius of curvature of spherical surface . It contains an accurately cut screw fitted into an accurately cut nut . The nut in turn is fixed on a metallic tripod framework . The three pointed legs of the tripod lie at the corners of an equilateral triangle . This plane which contains three pointed ends of legs is known as reference plane . The axis of screw is perpendicular to reference plane and passes through the middle of the equilateral triangle .

Scales : The spherometer contains two scales , head scale or circular scale which is attached to the top of the screw and and its circumference is divided into 50 or 100 equal divisions . The other is main or pitch scale which is attached to one of the legs of tripod such that it almost touches the rim of the circular scale . This scale has graduations of 1 mm .

Pitch : In spherometer the pitch is also defined as linear distance moved by its screw , when one complete rotation is given to the circular scale i.e.

$\displaystyle Pitch = \frac{Distance \; moved \; by\; Screw}{No.\; of\; complete \; rotations\; given\;to\;the\;screw} $

Least Count : The distance moved by the screw of the spherometer while circular scale is moved by just one division .

$\displaystyle L.C = \frac{Pitch}{No.\; of\; divisions\;on\;the\;circumference\;of\;the\;circular\;scale} $

Taking Measurements with a Spherometer : Take a glass slab as the base , keep the object (say glass sheet on it) . Turn the screw of spherometer upward . Place the spherometer on glass sheet and turn the screw downward till it just touches the glass sheet . Read the main scale reading and circular scale reading which falls inline with main scale . Remove the glass sheet and keep the spherometer back on the glass slab . Note down the main scale reading and circular scale reading . Thickness of the glass sheet is said to be the distance difference between two readings .

Zero error : Zero error cancels or is not important in a spherometer because the output is obtained by taking difference of primary and final readings .