The animated tutorial is here or at www.techtionary.com stepper-blog.swf

This is a historical view of telephone switching systems. Almon Strowger, a funeral director losing business to a competitor invented the step-by-step "stepper" first automatic telephone switching system patented in 1891. The only known movie of a real stepper is from the Alfred Hitchcock's "Dial M for Murder" - Courtesy Warner Bros. The TECHtionary tutorial also has a photograph of a step-by-step central office.
Here are the processes for step-by-step switching. The animated tutorial is available at http://www.techtionary.com under "S." The process begins telephone with customer number 1 is going to call customer number 2. The following animation demonstrates the steps that occur between Customer 1 dialing the phone number and Customer 2 hearing the phone ring. Here is a photograph from actual stepper CO-Central Office. This illustration shows the inside mechanics and outside appearance of a stepper, the unit that makes sense of the numbers the caller is dialing. This diagram represents the vantage point when a stepper is viewed from the top.
When the caller picks up the receiver, the telephone sends an electrical signal to a Central Office, the location the caller's unique line, along with many others, is identified with. When a stepper at the Central office identifies the line, it sends back dial tone.
This stepper sends a signal to another stepper, telling it the first number dialed by customer 1. That number represents the highest level in the hierarchy used to identify individual callers.

The Allotter sends a signal to the Group Selector, indicating the next step in the hierarchy of lines. This stepper performs both vertical and rotary stepping, as the rotary stepping identifies an intermediate step, and the vertical stepping represents the actual number dialed, a smaller step within the line identification.

The number assigned by the Allotter is sent to another stepper which acts as a Group Selector. The vertical stepper finds the next subgroup Customer 2 belongs to, and moves to the number assigned to this group. Next, within the same stepping unit, the rotary stepper moves further down into the hierarchy. The next four numbers are dialed, which frequently will be transferred to another central office entirely, continuing the same steps as demonstrated. When the final number is dialed, a signal is sent to a stepper acting as a Final Selector, and the call is connected.