Understanding Electrical Instruments: Absolute vs. Secondary

Electrical instruments are broadly classified based on how they measure electrical quantities. The question asks about Tangent galvanometer and Rayleigh current balance and which type of instruments they are examples of.

Let's look at the different types of instruments mentioned in the options:

• Secondary instruments: These instruments do not measure the electrical quantity directly in terms of the instrument's constants. Instead, they measure the quantity by observing the effect it produces (like deflection or torque) and are calibrated against a known standard. Their scale is marked in terms of the electrical quantity being measured. Examples include ammeters, voltmeters, wattmeters commonly used in laboratories and industries.
• Absolute instruments: These instruments give the value of the electrical quantity being measured directly in terms of the physical constants of the instrument itself. No prior calibration against a standard is required. By measuring the deflection produced and knowing the instrument's constants, the value of the quantity can be calculated using a formula.

Now let's consider the instruments mentioned:

• Tangent galvanometer: This instrument measures electric current based on the principle of tangent law of magnetism. The current passing through a coil produces a magnetic field, which interacts with the Earth's magnetic field. By measuring the angle of deflection of a compass needle, and knowing the Earth's magnetic field horizontal component (B_H), the coil's radius (r), and the number of turns (N), the current (I) can be determined using the formula: \( I = \frac{2r B_H}{\mu_0 N} \tan{\theta} \). All these factors (r, N, \(\mu_0\), B_H) are either physical constants or measured dimensions and known magnetic field components, allowing the current to be determined absolutely.
• Rayleigh current balance: This instrument measures electric current by balancing the magnetic force produced by the current against a known mechanical weight. It is a very precise instrument often used as a primary standard for current. The current is determined based on the physical dimensions of the coils, the force measured (which is balanced by a known mass and gravity), relating electrical units to mechanical units. This direct measurement based on fundamental physical principles and dimensions makes it an absolute instrument.

Both the Tangent galvanometer and the Rayleigh current balance allow the measurement of electric current directly from their physical properties and constants, without needing comparison with a pre-calibrated instrument. This characteristic defines them as absolute instruments.

Based on this, Tangent galvanometer and Rayleigh current balance are examples of absolute instruments.