Matt Pulzer addresses some of the general points readers have raised. Have you anything interesting to say? Drop us a line!

摘要

The design of the current adapter for multimeters by David L Jones in May's EPE is very good and a useful add-on to a DMM. It brings to light the principle that no measurement can be made without affecting the quantity measured by the act of measurement. The design presented shows how that effect can be reduced for the case of current measurement by inserting a series resistor and measuring the volt drop. This, however, set me thinking about the case where the current is measured by means of its magnetic field to minimise the effect of the measurement. My old AVO has a 50μA moving-coil meter, which drops 125mV for full-scale reading due to the resistance of the coil. If it was made from a superconductor operating with zero resistance, would it drop OV at 50μA?