---
title: "Direct, indirect, deflection and null methods"
subject: "Instrumentation and Measurement"
chapter: 1
key: "semester_4/instrumentation_and_measurement:fundamental-measurement-methods"
source: https://www.bctengineer.com/q/semester_4/instrumentation_and_measurement/fundamental-measurement-methods
site: BCT Engineering
---

# Direct, indirect, deflection and null methods

Instrumentation and Measurement · Chapter 1: Fundamentals of Measurement

- Asked in: year not given, 5 marks
- Video: [NULL-type and Deflection Type Instruments](https://www.youtube.com/watch?v=8EC37BT_FZ8) — CHEMISTRY TUTOR
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/instrumentation_and_measurement/fundamental-measurement-methods
- Whole chapter: https://www.bctengineer.com/md/semester_4/instrumentation_and_measurement/chapter-1

## Question

Explain the fundamental methods of measurement. Differentiate between the deflection method and the null method with one example of each.

## Short answer

Fundamental methods include direct comparison with standard, indirect via related quantities, and the null method where known quantity is adjusted for zero detector reading. In deflection method, output is read from pointer displacement (e.g., spring balance); in null method, a balanced opposition is achieved (e.g., Wheatstone bridge).

## Answer

Measurement is the process of comparing an unknown quantity with a standard of the same kind. The methods are grouped by how the comparison is made.

#### Fundamental methods
- **Direct method:** the unknown is compared directly with the standard or read on a calibrated scale. Example: length with a steel rule, mass on a beam balance.
- **Indirect method:** the quantity is found by measuring other quantities related to it by a known law. Example: power from $P = VI$, density from mass and volume.
- **Comparison method:** the unknown is compared with a known quantity of the same kind (a balance with standard weights).
- **Substitution method:** the unknown is replaced by a known standard that gives the same effect.
- **Null method:** the known quantity is adjusted until the difference is zero.

#### Deflection and null methods

| Point | Deflection method | Null method |
|---|---|---|
| Principle | Output shown as pointer deflection | Opposing known effect is adjusted until detector reads zero |
| Reading | From the scale | From the setting of the known quantity |
| Accuracy | Lower; depends on instrument calibration | Higher; depends on the standard and detector sensitivity |
| Loading effect | Draws power from source | Draws almost no power at balance |
| Speed | Fast | Slow, needs balancing |
| Example | Moving-coil ammeter, bourdon gauge | Potentiometer, Wheatstone bridge, deadweight pressure balance |

**Example of null method:** in a Wheatstone bridge the variable arm is adjusted until the galvanometer shows zero, and the unknown resistance is $R_x = R_2 R_3/R_1$.

**Example of deflection method:** a spring balance shows weight from the spring extension on a marked scale.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In the null method, what is done to obtain the measurement?
   A. The pointer deflection is read from a calibrated scale
   B. A known opposing quantity is adjusted until the detector reads zero
   C. The unknown is replaced by a standard giving the same effect
   D. Other related quantities are measured and put into a formula
   Answer: B. In a null method the known quantity is adjusted until the difference is zero, and the value is read from that setting. (Key point: Null method principle)

2. Why does the null method load the source less than the deflection method?
   A. It draws almost no power at balance
   B. It uses a faster detector
   C. It reads directly from a scale
   D. It has lower accuracy
   Answer: A. At balance the detector current is zero, so the source supplies almost no power; a deflection instrument draws power to move its pointer. (Key point: Loading effect)

3. Finding power from P = VI by measuring V and I separately is an example of which method?
   A. Direct method
   B. Null method
   C. Indirect method
   D. Substitution method
   Answer: C. The quantity is found from other quantities related to it by a known law, which is the indirect method. (Key point: Indirect method)

4. Which pairing of deflection and null methods is correct?
   A. Deflection: slow and more accurate; null: fast and less accurate
   B. Deflection: fast and less accurate; null: slow and more accurate
   C. Both have the same accuracy but null is faster
   D. Deflection: reads the known setting; null: reads the scale
   Answer: B. Deflection is fast but depends on calibration; null is slower because balancing is needed but its accuracy depends on the standard and detector sensitivity. (Key point: Deflection vs null accuracy)

## Related in Instrumentation and Measurement

- [Classification of measurement signals](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_and_measurement%3Aclassification-of-measurement-signals) — Instrumentation and Measurement, chapter 2
- [Limiting error in calculated power](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_and_measurement%3Apower-error-limiting-error) — Instrumentation and Measurement, chapter 1

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