A picture for Dave_J and other telephone guys:
From Bell System Technical Journal, October 1939,
http://www.alcatel-lucent.com/bstj/
http://www.alcatel-lucent.com/bstj/vol1 ... -4-547.pdf

From Bell System Technical Journal, October 1939,
http://www.alcatel-lucent.com/bstj/
http://www.alcatel-lucent.com/bstj/vol1 ... -4-547.pdf