A short length of circular glass tubing, of inside diameter large enough to contain a compass needle, stands with its axis vertical and has single or multiple loops of wire wound on it in vertical diametral planes.
"The Earliest Electromagnetic Instruments"
Robert A. Chipman
First, by their diameters at the pitch circle or pitch diameter and the number of teeth they contain; second, by the number of teeth in the wheel and the pitch of the teeth; and third, by a system known as diametral pitch.
"Modern Machine-Shop Practice, Volumes I and II"
Joshua Rose
What is the diametral pitch?
"Modern Machine-Shop Practice, Volumes I and II"
Joshua Rose