Standard Pony Relay
For general use in single Morse operation. Common telegraph relay with pair of adjustable magnets,
armature, "goose-neck" contact carrier, all mounted on a wood base.
Type 4-B, the oldest model is mechanically larger than the later 4-C and 4-D,
and has a higher resistance for a given operating current. The 4-B type was originally made in 35 and 150 ohm resistance,
but 150 ohm only is now furnished, the other being superseded by 4-C 25 ohms.
The 4-B type must be used wherever space and resistance will permit.
Telegraph lines were copper or iron wire. Hence after a certain distance,
the resistance of the wire attenuated the current in the line, rendering the sounder insensitive.
To extend the useful range of the telegraph, relays were used.
The relay is basically an electromagnet as is the sounder. However, the solenoids of the electromagnet
usually have more turns of wire and therefore are quite sensitive. When the operator on the sending end pressed his key,
the armature completed a new circuit.
Relays had the same life cycle as keys and sounders. The relay is the mid 19th century electromechanical version
of the 20th century amplifier.
Skirrow Patent Dec. 10, 1901
Postal Telegraph Company owned the "Skirrow" patent which involved
the use of a rack-and-pinnion mechanism to adjust the coil-to-armiture distance to allow the
sounder to be made more or less sensitive to the tiny currents on long telegraph lines.