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