Strowger switch
![Western Electric 7A Rotary, friction drive (Bird-cage), No. 7001 Line Finder. Note the driven bevel gear on the right-hand side; this type has a steady rotary motion and does not employ an electromagnet for stepping.](/Images/godic/202502/13/Uniselector_Stepper_detail3552.jpg")
![British Strowger exchange, BPO 2000-type equipment](/Images/godic/202502/13/Catford_Strowger_Switch3552.gif")
![Télégraphie, Systeme Strowger](/Images/godic/202502/13/Télégraphie,_Systeme_Strowger3552.jpg")
The Strowger switch was the first commercially successful electromechanical stepping switch telephone exchange system. It was invented by Almon Brown Strowger, and first patented in 1891. Because of its operational characteristics it is also known as a step-by-step (SXS) switch.