EFFECT OF PRIMARY COIL SIZE ON EDDY-CURRENT DECAY RESISTIVITY MEASUREMENTS

Primary coil dimensions can influence the accuracy of resistivity measurements made by the eddy current decay (ECD) method. Experimental results revealed that when the length of a 22.5 mm i.d. primary coil is changed from 60 to 20 mm, the corresponding measured values of resistivity increase by 4-5%. The effect occurs when there is an insubstantial background field at the sample ends. ECD measurement accuracy is better than 1% when the primary coil length to inside diameter ratio is larger than 2.2 and the sample diameter is 44% or less of the primary coil i.d. The investigation included measurements at room temperature, 77 K and 4.2 K on samples having residual resistivity ratios of 153, 340, 1010 and 7790.