Editing Blue box (section)

===Automating dialing===
Local [[plain old telephone service]] works by watching the voltage on the telephone lines between the telephone company's [[telephone exchange|exchange office]] and the customer's telephone. When the phone is on-hook ("hung up") the approximately 48{{nbs}}[[volt]] electricity from the exchange flows to the phone and is looped back without passing through the handset. When the user picks up the handset, the current has to flow through the speaker and microphone in it, causing the voltage to drop to under 10{{nbs}}V. This sudden drop in voltage signals the user has picked up the phone.

Originally, all calls were routed manually by an operator who would look for small [[light bulb]]s that would illuminate when a subscriber picked up the phone to make a call. The operator would connect a handset to the line, ask the user who they were calling, and then connect a cable between two [[Phone connector (audio)|phone jacks]] to complete the call. If the user was placing a long-distance call, the local operator would first talk to an operator at the remote exchange using one of the trunk lines between the two locations. When the local operator heard the remote customer come on the line, they would connect their local customer to the same trunk line to complete the call.

The calling process began to be automated from the earliest days of the telephone system. Increasingly sophisticated [[electromechanical]] systems would use the changes in voltage to start the connection process. The [[rotary dial]] was introduced around 1904 to operate these switches; the dial repeatedly rapidly connects and disconnects the line, a process known as [[pulse dialing]]. In common systems, these periodic changes in voltage caused a [[stepper motor]] to rotate one position for each pulse of a digit, with longer pauses to switch from one rotary switch to another. When enough digits had been decoded, typically seven in North America, connections between the rotors would select a single line, the customer being dialed.

The idea of using changing voltages to complete the call worked well for the local exchange where the distance between the customer and exchange office might be on the order of a few kilometers. Over longer distances, the [[capacitance]] of the lines filter out any rapid changes in voltage and dialing pulses do not reach the remote office in clean form, so that long-distance calls still required operator intervention. As telephone use grew, long-distance calling in particular, telephone companies were increasingly interested in automating this type of connection.