- Practice · 5 marks
Direct, indirect, deflection and null methods
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).
Explain the fundamental methods of measurement. Differentiate between the deflection method and the null method with one example of each.
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 , 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 .
Example of deflection method: a spring balance shows weight from the spring extension on a marked scale.
✓ Answer checked by Claude Opus 5.5 (high effort)
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