Contact resistance drops at the leads
Contact resistance refers to the resistance that occurs at the interface between two conductive materials. It is typically present at the junction between the leads and the test object. In the measurement of low resistance, contact resistance can have a significant impact on the accuracy of the measurement. However, in this question, it states that "contact resistance drops at the leads," which means that the contact resistance is reduced or minimized.

Explanation:
Contact resistance occurs due to imperfect contact between two conductive materials. It can be caused by factors such as surface roughness, contamination, oxide layers, or poor alignment. When measuring low resistance, contact resistance can introduce additional resistance that is not part of the actual resistance being measured, leading to measurement errors.

Importance of accurate low resistance measurement:
Accurate measurement of low resistance is crucial in various applications, such as in the testing of electrical connections, grounding systems, or the evaluation of material properties. Any errors in the measurement can result in incorrect assessments, leading to potential safety hazards or inefficient performance.

Impact of contact resistance:
Contact resistance can be a significant source of error in low resistance measurements. It can introduce additional resistance that is not part of the actual resistance being measured. This extra resistance can make the measured value higher than the true resistance, leading to inaccuracies in the measurement results.

Minimizing contact resistance:
To minimize contact resistance, it is important to ensure good contact between the leads and the test object. This can be achieved by using proper cleaning techniques, such as using alcohol wipes or abrasive pads to remove any contaminants or oxide layers. Additionally, applying a small amount of conducting grease or conducting paste at the contact interface can help improve the contact and reduce contact resistance.

Conclusion:
In summary, contact resistance can be a significant source of error in the measurement of low resistance. However, in this question, it states that "contact resistance drops at the leads," indicating that the contact resistance is reduced or minimized. Therefore, option A is not a source of error in the measurement of low resistance.