Coil construction: Which change definitely increases the DC winding resistance of a coil (ignore core effects on resistance)?

Introduction / Context:
A coil’s DC winding resistance affects copper loss and temperature rise. Designers control this resistance by choosing conductor gauge and number of turns. Understanding which parameters increase resistance is key for balancing efficiency, size, and cost in inductors, transformers, and electromagnets.

Given Data / Assumptions:

• We focus on DC winding resistance only (not reactance).
• Material of the wire is copper unless otherwise stated; resistivity is constant for this comparison.
• Core changes do not alter the copper’s ohmic resistance.

Concept / Approach:
Wire resistance R = ρ * (ℓ / A), where ρ is resistivity, ℓ is conductor length, and A is cross-sectional area. Increasing turns increases the length ℓ, raising R. Using thinner wire decreases A, also raising R. Core material does not appear in this formula and thus does not affect winding resistance directly.

Step-by-Step Solution:

Verification / Alternative check:
Practical test: measuring coil resistance with an ohmmeter before/after adding turns or switching to a smaller gauge consistently shows higher resistance; swapping the core alone leaves ohmic resistance unchanged.

Why Other Options Are Wrong:

• Changing the core material: affects inductance and losses like hysteresis/eddy currents, not winding DC resistance.
• Using thinner wire only or increasing turns only: each individually increases R, but the most complete correct statement is that either change will increase R.

Common Pitfalls:

• Confusing inductance changes with resistance changes.
• Ignoring the strong effect of conductor cross-section on R.

Final Answer:
increasing the number of turns or using thinner wire