viii PREFACE 



written. I am aware that by doing so I have neglected priorities, 

 but such may be permissible in a book which is not intended as an 

 historical source book. The reader will, in general, find the original 

 source for any one method by following up the references. Only 

 references in ordinary scientific and technical periodicals or text- 

 books are quoted. References to semipublic material, such as theses 

 or technical reports of research-sponsoring agencies, are used only 

 occasionally. Experience shows that a great part of such semipublic 

 material eventually finds its way into the regular literature. Also, 

 patents are quoted occasionally. There is a wealth of information in 

 the patent literature, but it would require a fairly large organization 

 to make it available in a practical form. 



A problem which defies solution at the present time is that of 

 units and dimensions. It would simplify matters considerably if a 

 book could be written with the use of one system throughout, e.g., 

 the mks system, 1 in particular since almost everybody concedes the 

 superiority of a metric system. Unfortunately the physicist uses 

 the cgs and the mks systems, the medical research worker frequently 

 a hybrid system, the engineer, in particular the mechanical engineer, 

 and many industries the English (inch-pound, etc.) system. I see 

 no way out of this difficulty. The complexity of the situation is 

 increased by a desire of certain authors to invent new, unnecessary 

 names for dimensions ("dyne-five newton," "pharos") and, even 

 worse, to form new derivations from such names ("pragilbert/ 

 weber"). The situation becomes intolerable with the introduction 

 of the "mho" (ohm" 1 ) which, fortunately, has not (or not yet) given 

 rise to the "tlov" (volt -1 ). I wish to state, with due humility, that 

 the essence of any science is simplicity, and that it must be con- 

 sidered an asset to derive the infinite number of dimensions from a 

 combination of the smallest possible number of basic units. How- 

 ever, until agreement has been reached on the question of units and 

 dimensions, this book reflects the deplorable status of the art. 



I should like to express my gratitude to the Massachusetts Insti- 

 tute of Technology, in particular to Dr. I. W. Sizer, head of the 

 department of biology, who has freed me from my teaching duties 

 for one year. 



I am particularly thankful to the Office of Naval Research, which 

 has sponsored the greater part of the research activity involved in 

 writing this book and provided interest and help that have been a 



1 Differences exist also in the mks system. The "k" in the American mks 

 system is the unit of mass, in the European mks system it is frequently the 

 unit of force. 



