This article was published in two parts in the British journal Biomedical Engineering, which

apparently no more exists.

Part 1 in Vol. 10, No. 8, August 1975, pp. 300-302

Part 2 in Vol. 10, No. 9, September 1975, pp. 342-345 + 349

A correction has been introduced in the formula (10) in Part 1

Addresses mentioned are no more valid