engineering

Local attraction

Local attraction is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Local attraction rather than just read about it. In short: While compass surveying, the magnetic needle is sometimes disturbed from its normal position under the influence of external attractive forces. Such a disturbing influence is called local attraction.

Key takeaways

Local attraction belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Local attraction to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Local attraction from memory before moving on to harder problems.

Reference excerpt

While compass surveying, the magnetic needle is sometimes disturbed from its normal position under the influence of external attractive forces. Such a disturbing influence is called local attraction. The external forces are produced by sources of local attraction which may be current carrying wire (magnetic materials) or metal objects. The term is also used to denote amount of deviation of the needle from its normal position. It mostly causes errors in observations while surveying and thus suitable methods are employed to negate these errors.

Sources The sources of local attraction may be natural or artificial. Natural sources include iron ores or magnetic rocks while as artificial sources consist of steel structures, iron pipes, current carrying conductors. The iron made surveying instruments such as metric chains, ranging rods and arrows should also be kept at a safe distance apart from compass.

Detection Local attraction at a place can be detected by observing bearings from both ends of the line in the area. If fore bearing and back bearing of a line differ exactly by 180°, there is no local attraction at either station. But if this difference is not equal to 180°, then local attraction exists there either at one or both ends of the line.

Remedies There are two common methods of correcting observed bearings of the lines taken in the area affected by Local Attraction. The first method involves correcting the bearing with the help of corrected included angles and the second method involves correcting the bearing of traverse from one correct bearing ( in which difference between fore bearing and back bearing is exactly equal to 180°) by the process of distribution of error to other bearings.

References

Worked examples

Example 1 — a first encounter with Local attraction

Start with the simplest possible case. Write down what Local attraction claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Local attraction before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Local attraction ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Local attraction

In research: Local attraction appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Local attraction in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Local attraction is common in secondary-school and first-year university syllabi. It links to neighbouring topics Civil engineering, Surveying, so understanding it makes those chapters shorter.
In everyday life: Look for Local attraction outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Local attraction in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Local attraction means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Local attraction out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Local attraction in simple terms?

While compass surveying, the magnetic needle is sometimes disturbed from its normal position under the influence of external attractive forces. Such a disturbing influence is called local attraction.

Why does Local attraction matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Local attraction?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Local attraction.