A thermocouple cylinder-head temperature indicator requires leads with specific resistance typically 2 or 8 ohms.

• A. The resistance value specified on the indicator (usually 2 or 8 ohms)
• B. The lead length
• C. The material of insulation
• D. The color coding of the leads

Answer: (A)

This indicator is designed to read a temperature using a resistance in the circuit, so it is calibrated for a specific total lead resistance. The leads must add exactly the resistance the instrument expects (usually 2 or 8 ohms) because any extra resistance in the leads changes the circuit and throws off the temperature reading. Lead length can affect resistance, which is why the specified resistance is important—it guarantees the reading stays accurate. The insulation material and the color coding of the leads don’t determine the electrical value the instrument relies on, so they don’t affect the measurement in the same way.