Zither telephone switch explainer by Keith

Keith’s Zither Switch

Highlights

Switch category: Linear and Rotary motions

Inventors: A.E. Keith, F. Lundquist and the Erickson brothers, employed by the Strowger Automatic Telephone Exchange Company.

Important dates: Patent US 540,168 granted 1895. Installed in 1894 at La Porte (Indiana) exchange, replacing the first Strowger switch.

Legacy: This clever switch had limited success and was removed from the La Porte office after one year. However, it demonstrated the abilities and inventiveness of Keith and the Ericksons. It utilized concepts that were later repurposed in other switches.

Switch details

Quoting from Keith’s patent, “A very desirable form of (automatic) switchboard is made by arranging a series of wires parallel with each other in such a position that each connector (shaft) is capable of making and breaking electrical connection with each and every one of them.”

“Shaft movement is in two directions. The shaft is capable of a rotary and a longitudinal movement." (simplified)

Most authors call the device a “Piano Wire switch” or “Zither switch”, for evident reasons. Let’s zoom in to clarify how it works. All the pictures in this section are from the London Science Museum collection.

Alexander Keith image of his Zither or Piano Wire switch from 1894

Fig 1, Original Zither switch with seven individual connecting shafts and fingers

All movements in the mechanism result from either the action of electromagnets or the release of springs. This is a one-wire-per-talking-path switch. However, there are seven shafts with connecting fingers so seven separate talking paths may be established independently and simultaneously (for this model). This was innovative and a new direction in switch design.

Referring to the image below, observe the means to rotate and translate the connecting shaft. This working model does not follow the patent in every detail so expect some deviations if comparing.

image zoomed of Zither telephone switch showing laterial and rotary movement means

Fig 2, Close up showing longitudinal and rotary motion parts

For linear translation, the design incorporates a round component with circumferential grooves. An electromagnet engages it and moves the shaft linearly to the right for each step. The amount of steps depends on the target taut wire to connect to.

For rotary motion, a pawl engages the ratchet cylinder in the figure. Each step causes the shaft to rotate in the same direction such that only ONE finger is touching and connecting to a taut wire.

The resetting mechanism is based on two helical springs returning the shaft to its neutral position activated by a release magnet. In the first picture above, notice the spring at the right end of each shaft for powering the shaft's return.

Zither or piano-wire telephone switch showing laterial and rotary motion ratchets

Fig 3: A side view illustrating the mechanism and the ten fingers per shaft

In the above figure, note that each finger on a given connecting shaft has a distinct rotational orientation. By design, the fingers are offset by ~32 degrees on the shaft so that only one of ten may contact a taut wire at a time per mechanism. One angular rotation setting (call it the 11th) is for releasing, where no finger makes wire contact so the shaft may return to neutral.

zither telephone switch zoomed in view showing laterial and rotary motion means -- how it works

Fig 4, Observe the different longitudinal positions of the four shafts

Interestingly, despite its complexity and differences from the first Strowger switch, this design seamlessly replaced the original at the La Porte exchange. The patent describes a common exchange battery for all switch operations and not powered by the subscriber’s battery. This was a plus.

The subscriber still needed to control the switch motions using three push buttons (#T/U, release). The Tens key moves the shaft longitudinally and the Units key rotates the fingers. The patent was limited to 90 lines, omitting numbers 10, 20, 30 and so on out of 99. The rotary dial was not yet invented, but very close to being so.

A Closer Look at the Fingers

The illustration below, with two distinct shafts, clearly shows how a single finger can establish a connection with a specific taut wire through a blend of longitudinal and rotary movements. All the fingers on a shaft are essentially the same terminal; one side of a 2-part connection. A selected taut wire is the second terminal of the connection.

The fingers are distributed such that no more than one out of ten is able to make contact with a taut wire at the same time. However, in the figure below the fingers are misaligned after many years of handling.

zither telephone switch wiper making connection to piano wire

Fig 5, Connecting fingers on the shafts

The wire to finger interface is very delicate. Just a small misalignment of the wire or finger could cause a misconnect. This is one reason why the switch was discontinued; it was not sufficiently robust and had maintenance headaches.

Enduring Legacy of the Design

Despite the switch’s downsides, the linear and rotation means was a breakthrough idea. Keith (and his partners) was soon to create another invention; an improved Strowger switch, and they applied what was learned from the Zither to the new invention.

image explanatio of zither switch simularity to strowger switch motion means

In the figures above it’s apparent that the Zither’s shaft motion means was repurposed with some modifications for a new and improved Strowger style switch. Sure, there are many differences between the two switches, but the essential motion means survived.

Keith's innovative repurposing stands as a testament to his creativity and it was the start of a long and prosperous career in switch and exchange design.