vernier theodolite and its parts

 

VERNIER THEODOLITE

·        The Vernier Theodolite is also known as Transit theodolite, In a Vernier theodolite the telescope can be rotated in a vertical plane.

·        The main parts of a theodolite are the following.

Levelling Head:

·        Levelling head is the base of the instrument, it has provision to attach the instrument to a tripod stand while in use and attach a plum bob along the vertical axis of the instruments.

·        The upper head plate of the levelling head is also known as “Tri batch”, has three arms each with a foot screw.

·        The lower part is known as “Trivet”, has a central hole and a hook to which a plum bob can be attached.

Lower Plate:

·        The lower plate is also known as the circle plate is an annular horizontal plate with a bevelled graduated edge fixed to the upper end of a hollow cylindrical part.

·        Horizontal angles are measured with this plate.

·        The diameter of the lower plate is sometimes used to designate the instrument.

Upper Plate:

·        The upper plate is also a horizontal plate of a smaller diameter attached to a solid ,vertical spindle.

·        The bevelled edge of the horizontal part carries two Vernier’s on diametrically opposite parts of its circumference.

·        These Vernier’s generally designated “A” and “B”, they are used to read fractions of the horizontal circle plate graduations.

 

Plate Level:

 

·        The plate level is a spirit level with a bubble and graduations on the glass cover.

·        The spirit level can be adjusted with the foot screw of the levelling head.

·        The bubble of the spirit level can be moved with the foot screws of the levelling head.

 

Standard (or) A-Frame:

·         Two standards in the shape of the letter “A” are attached to upper plate.

·        The horizontal axis of the instrument is attached to these standards.

·        The A-frame supports the telescope and the vertical scale.

 

Telescope:

·        The telescope is a vital part of the instrument, it enables one to see stations that are at great distances.

·        The essential part of a telescope are the eye piece diaphragm with cross hairs, object lenses and arrangements to focus the telescope.

 

Vertical Circle:

·        The vertical circle is a circular plate supported on the horizontal axis of the instrument between the A-Frames.

·        The vertical circle moves with the telescope when it is rotated in a vertical plane.

 

Tripod:

Tripod is an accessory which is used to place theodolite, the lower plate of theodolite will be fixed  to tripod with the help of screw.

 

Plum bob:

·        A heavy plum bob on a good string with a hook at the end is required for centring the theodolite over a station.

·        The plum bob is fixed to the hook (or) other device projecting from the centre of the instrument in a central opening in the levelling head.