📏 Absolute and Secondary Measuring Instruments
Electrical instruments can be classified into absolute and secondary instruments based on how they measure the quantity.
🔹 1. Absolute Instruments
Definition:
Absolute instruments measure electrical quantities directly in terms of the instrument's physical constants. They do not require calibration from any standard instrument.
Working Principle:
The measurement is calculated using a mathematical formula involving physical constants like spring constant, dimensions, and deflection.
Examples:
- Tangent Galvanometer
- Rayleigh’s Current Balance
- Absolute Electrometer
| Feature | Description |
|---|---|
| Accuracy | High theoretical accuracy |
| Usage | Laboratory use |
| Calibration | Not needed |
| Construction | Complex and bulky |
🔸 2. Secondary Instruments
Definition:
Secondary instruments indicate the electrical quantity by a pointer or display. They must be calibrated using absolute instruments or standards.
Working Principle:
The deflection shown is proportional to the electrical quantity, and it depends on calibration for accuracy.
Examples:
- Ammeter
- Voltmeter
- Wattmeter
- Energy Meter
| Feature | Description |
|---|---|
| Accuracy | Depends on calibration |
| Usage | Used in field and industry |
| Calibration | Must be calibrated |
| Construction | Compact and easy to use |
⚖️ Key Differences
| Aspect | Absolute Instruments | Secondary Instruments |
|---|---|---|
| Measurement | Direct using physical constants | Requires calibration |
| Use | Laboratory only | Industrial and practical use |
| Calibration | Not needed | Essential |
| Complexity | Complex, bulky | Compact, user-friendly |
| Examples | Tangent Galvanometer, Current Balance | Ammeter, Voltmeter, Wattmeter |