I watched LAaRT on PBS OC tonight and was introduced to this guy who custom crafts guitars. On the episode they showed him talking about a wafer switch that he said came from a telephone system. This switch is like a rotary switch turned on edge and a lever added to move the contacts to the different positions. I used to see these in complex switching systems such as a tube tester, where the different pins have to be assigned to different elements such as filament, grid and plate. These switches hold up well under intermittent usage such as a tube tester, but they would not last long in a switchboard where they get heavy daily usage. The kind of switch that is used in the switchboard was especially designed by the telephone company for high usage. These switches were made by such companies as Western Electric and Kellogg. I imagine that they would be very expensive to build today. Nowadays the switches use electronics to sense touch instead of physically switching contacts. And the switches used for very high usage are designed for minimal contact wear. One way to do this is to use reed switches, which have their contacts sealed in glass and are activated by a magnet that moves close to it.
The telephone system used leaf switches. The leaves have contacts that are made for heavy use, and when the contacts close, they wipe across each other so any oxidation is rubbed off. Some leaf switches have more than one contact per leaf, which helps increase reliability. The leaves can be held together by screws, which allows the leaves to be removed and replaced. These switches are very rugged and seldom fail, unlike the wafer and rotary switches which sometimes become damaged during normal use. The leaves are actuated by a disk attached to the handle. The disk may have one or more cam lobes that move across the leaves, opening and/or closing them. The arrangement of the leaves, spacers and other mechanical paraphernalia allows for very sophisticated and complicated switching arrangements, such as make before break, delayed make, etc. Springs can give it momentary contact, etc.
I have some old leaf switches most likely from the telephone system in some box somewhere in my stuff in the garage. If I can find them, I’ll have to take some photos – I looked through Google images and found very little in the way of good pictures.
Back to experimenting…
