the line from the telephone substation 11 to the central office 13 is totally inactive no line current i l flows because the telephone is 34 on hook 34 and the hook switch 15 is open in addition the switch 17 is closed only during the hold condition to be discussed no ringing signal voltage v r is present during the idle condition and the control terminal 19 connected to what is commonly called the 34 a conductor 34 is ungrounded the a conductor terminal 19 is connected to the three switches 21 23 and 25 within the telephone station set switch 21 is normally closed and is opened only when the hold key 22 on the telephone is depressed switch 23 is closed whenever the line select key 24 is depressed key 23 in the illustration of fig1 is used to select the second of five lines to the central office finally switch 25 is closed whenever the telephone is off hook thus terminal 19 is not grounded in the idle condition because at least switch 25 is open the ringing condition occurs when ringing voltage v r is applied to the line at the central office 13 indicating an incoming but still unanswered call the line to the central office.