Essentially, a compass is a little gadget with a magnetic needle, a graduated circle, and a line of sight. If you point a compass directly at a line, the magnetic needle will point at the magnetic meridian, and the angle between the line and the meridian may be read at the graduate circle. A compass would not be able to directly measure the angle between the two lines. This prismatic compass, together with the magnetic compass, are the two primary instruments used in the series for magnetic bearing measurement.
Prismatic compasses are small and bendable enough to be held in one’s hand. The main component of a prismatic compass is a prism, which ensures accurate readings. Charles Schmalcalder patented his invention of the prismatic compass in 1812. Before the turn of the century, prismatic compasses were not mass-produced.
You may use a prismatic compass in two ways: either by holding it in your hand or mounting it on a tripod. The ability to see and read the compass at the same time is one of its primary benefits. This explains why it sees such heavy application in the field of land surveying.
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Prismatic compass: Components
Metal cylinder container
The whole compass, including the casting or body, is shielded by the cylinder-shaped case, which has a diameter of 8 to 12 cm.
Pivot
The magnetised needle is supported by a pivot located at the compass’s centre.
Magnetic needle
The instrument’s accuracy depends on the magnetic needle. The angle of a line’s deviation from the magnetic meridian may be determined by observing which end of the needle is pointing towards the north or south pole when the needle is hung freely.
Graduation circle
An aluminium-graded ring having degrees from zero to three hundred and sixty-five and a magnetic needle for measuring any bearing on a line.
Prism
Prism stands in direct opposition to the object vane. A cover made specifically for prisms keeps debris and moisture out of the opening.
Object vane
There are two more vane types, the prism vane, and the eye vane; however, the object vane is completely contrary to both. The horsehair or thin black wire on the object vane serves as a sight line for the item.
A glass cover
You can see the needle and graded ring through the top of the instrument cover.
Sunglasses
These are used to bisect a bright item in half.
Reflecting mirror
When bisection is applied to the object vane, it is possible to get a picture of a target that is either above or below the instrument’s viewing plane.
Spring break
Before obtaining a reading, the light spring break connected to the box is pressed inward until it comes into contact with the wedge of the ring, dampening the oscillation of the needle and bringing it to rest fast.
Prismatic compass: How to use
- Centring– To achieve proper centring, the compass should be fastened to the stand and placed above the station. The compass legs of the tripod stand may be adjusted inward and outward or around the circle to bring it back to the centre. A stone placed in the centre of the compass may be used to verify the plumb’s ability to centre.
- Levelling– While exact levelness isn’t required for using a compass to survey, it should be close enough so that the magnetic needle may float freely. The bubble should then be placed precisely in the centre of the circle designated for it on the level.
- Focusing the prism– Sliding the prism’s attachment up and down the graded circle brings the readings into sharp focus in a prismatic compass.
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Prismatic compass: Uses
Whether you’re orienting a chart or a plane table, conducting a survey, or navigating, a prismatic compass is an indispensable tool. In crude surveys, when speed is more important than accuracy, a prismatic compass is the tool of choice. The employment of geologists and surveyors is commonplace for taking accurate measurements of degrees and angles. To determine the direction of a magnetic meridian, a prismatic compass is used.
Prismatic compass: Advantages
The best part about using a prismatic compass is that you don’t have to move your head around to read the circle or sight the item. You don’t have to move your eye from its present position to see the item and make sense of the circle.
- Their portability and minimal weight are two of their best features.
- Fixing it on a space station requires fewer adjustments.
- A mistake in one survey won’t significantly alter the results for any other lines.
Prismatic compass: Disadvantages
- Incapable of being done in areas where magnetism is present.
- In order to identify iron ore, professional help is required, particularly from those knowledgeable in the field of geology.
- There is a high potential for inaccuracy while getting a bearing since it is difficult to keep the compass completely motionless.
- Long-distance surveys cannot be done using a prismatic compass.
FAQs
How does the prism work in a prismatic compass?
The prism is held in place by a mount that can move up and down between slides that are attached to the outside of the case. This up-and-down movement is meant to help you focus by giving you a chance to adjust.
Which two kinds of compasses are there?
Most of the time, two types of compasses are used: prismatic compasses and surveyor compasses.