Parts of Prismatic Compass

Prismatic Compass
The term “prism-based compass” refers to a compass that uses a prism to determine the direction, waypoints, and bearing of travelling while also accounting for angles between them. It is compact and typically held in the hand. The surveyor’s compass and prismatic compass are the two primary magnetic compasses included in the collection to assess magnetic bearing.

Parts
The following are parts of the prismatic compass:

Cylinder-shaped metal box

The compass and its whole casting or body are protected by a cylindrical box with a diameter of 8 to 12 cm.

Pivot

The pivot, which supports a magnetic needle that is suspended freely, is situated in the centre of the compass.

Magnetic needle

The instrument’s beating heart is the magnetic needle. This needle, when freely hanging on any support, always maintains its two ends pointed in the direction of the north-south pole, measuring the angle of a line from the magnetic meridian.

Graduation Circle

This is a magnetically connected graded ring made of aluminium that is marked with 0° to 360° to measure all conceivable line bearings.

Prism

The graduation of the ring can be read using a prism, and a compass reading can be taken precisely. It is positioned directly across from the object vane. A prism cap covers the prism hole to keep out dust and moisture.

Object vane

The prism and eye vane are diametrically opposed to the object vane. The object vane is carrying black thin wire or horsehair to match up with the object sight.

An eye-vane

To separate an object from silt, an eye-vane is a fine silt with a hole for the eyes in the bottom.

Glass covering

It encloses the instrument box from the top so that the graduated ring and needle are visible from that angle.

Sunglasses

When a luminous item needs to be divided, they are utilized.

Reflector mirror

When bisection is applied to the object vane, it is utilized to obtain an image of an object that is situated above or below the instrument level.

Spring break

By gently pressing the brake pin in, the light spring break linked to the box is brought into contact with the wedge of the ring, dampening the movement of the needle before recording the reading and rapidly bringing it to rest.

Working
A prismatic compass can be used while being held in the hand. For better precision, it is typically mounted on a small tripod with a vertical spindle that has a ball and socket system and to which the compass is screwed. The compass can be readily positioned with this setup. These three steps are necessary for it to function.

Centering
Leveling
Taking note of the bearing
Centering

A small stone is dropped beneath the compass’s center until it hits the top of the peg designating the station, positioning the compass’s center vertically over the station point.

Leveling

The compass is leveled using a ball and socket mechanism, allowing the graduating ring to swing quietly and freely. A circular pencil may be used to test it against the compass box.

Learn more about magnetic intensity here.

Taking note of the bearing

After leveling the instrument and centering it over the station, adjust the prism’s height such that, when viewed through the prism, the scales on the ring are plainly visible.

The range rod at the forward station must be plainly visible before turning the compass box. Take the reading where the hairline seems to cut the graduation ring after using the brake pin to stop the ring and bring it to rest.

Why is zero marked south in the prismatic compass?
The graduations flow clockwise from the south end of the needle, which is 0° to 360 °, in the prismatic compass because the circle is read at the observer's end rather than the target's end. While the graduations on a surveyor's compass travel counterclockwise from the north. As a result, the south point of the needle and the zero degrees of the graduations line up.

Closing error
Closing error is the distance by which the end of the traverse falls short in coinciding with the starting point of the traverse.

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