In this paper an analysis of the performance on both internally- and externally-terminated current transformers is presented. Factors limiting the I-t, droop, peak pulse current, frequency response and mean dc current performance of these instruments is discussed and trade-offs examined. Experimental results which demonstrate the behaviour of inexpensive ring-type designs over a broad spectrum of frequencies are examined and compared with those achieved with internally-terminated structures. It is shown that some externally-terminated ring-type designs can produce a good and versatile performance in the measurement of pulse currents with frequency components below 0.1 Hertz and to several hundred kilohertz. Internally-terminated designs extend the performance by two to three decades of frequency for applications where high-frequency harmonics are to be measured accurately.