PL-2 Identification

NW Mailing List nw-mailing-list at
Fri Aug 28 14:32:13 EDT 2015

>From my interviews with older supervisors from the PRR Signal Department,
the PRR did NOT use the PL-2.

And from the evidence which existed until recently, the N&W did NOT use the
PL-3, at least until some VERY late replacements (e.g. Singer.)  They
probably bought the PL-3 equipment only because the PL-2 was no longer in
production at US&S.  I have heard the Pocahontas Div also had some PL-3

As far as I can determine, the N&W was the only road which bought the
PL-2.  That makes me wonder if it was developed specifically for the N&W.
The optics of the PL-2 have twice the horizontal light beam "spread" (18
degrees)  of the PL-1 and PL-3, which would have suited the N&W's somewhat
curvy alignment quite nicely.  Most of the N&W's PL-1 equipment was changed
out with PL-2 as it rusted out or otherwise failed.  The last PL-1s I saw
in service on the N&W were at Randolph Street, Roanoke, in the early 1990s.

Attached is a photograph of the three iterations of PL equipment.  The PL-1
is on the bottom; the PL-2 is in the middle; the PL-3 is at the top.  Each
has its own unique optical system.

The optical system of the PL-1 consisted of a clear plano-convex lens with
stepping (concentric rings) and a cast prism in the middle, utilized the
conical cover glass tinted in a color called No-Viol Yellow, and had a
mirror above the lamp, set at 45 degrees, to give the downward indication.
The PL-1 went into production in either 1922 or 1923.

In the optical system of the PL-2, the plano-convex lens was dispensed
with, the deflecting prisms were cast into the back surface of the lens
(which was cast in No-Viol glass,) and the mirror was directly behind the
lamp (with the possibility of phantom indications being eliminated by the
insertion of a honeycomb "phan-kill" directly behind the lens.)  Also note
that the conical cover glass was eliminated on the PL-2.  The prisms cast
onto the lens provided for a 20 degree downward deflection, which could be
enhanced by the insertion behind the lens of an additional 15 degree "cat's
eye" prism of 2.25" diameter, retained in place by a phosphor-bronze
retaining clip.   (In the attached photo, the "cat's eye" downward prism is
visible in the upper part of the  illuminated area.)  The earliest drawings
I have for the PL-3 are dated 1937.

The optical system of the PL-3 utilized a clear, flat  5 3/8ths inch lens
set at a 15 degree angle within the housing itself, and the downward
deflecting prisms were cast into this lens.  Again, the mirror was directly
behind the lamp, the "phan-kill" honeycomb was used, and a conical
cover-glass was used on the front.  The earliest drawings I have for the
PL-3 are dated 1941.

The PL-1 and PL-2 equipment both use 11.5" visors.  The PL-3 equipment
utilizes a 14" visor.  The PL-1 has a single wing bolt through the side
plates; the PL-2 and PL-3 use two wing bolts.  The main body castings of
all three versions differ significantly.

As for lamp bases, the PL-1 had its lamp receptacle bolted to the bottom of
the cast iron housing;  the PL-2 and PL-3 equipment employed a lamp
receptacle mounted horizontally at the center of the lens, carried on a
small stainless steel bracket brought up from the bottom of the housing, in
an inverted V-shape.  Both arrangements provided for the lamp filament to
be within 1/64" of the optically precise focal point behind the lens.  (The
1/64" criterion is the meaning of the term "signal precision.")

The function of the conical cover glass was shed snow.  Its tip was frosted
to eliminate the "hot spot" when the signal was viewed at night.  No-Viol
(i.e. "no violet") glass was developed as the result of the work of Dr.
Churchill, Corning Glass' head optical engineer, based on his earlier
attempts to improve the fog-penetrating qualities of the lights on European
light houses.  Churchill found that if the violet is removed from a beam of
light, the beam does not scatter so badly in fog.  No-Viol glass does not
transmit violet.  The Father of the PL Signal, Alexander Holley Rudd, Chief
Signal Engineer of the PRR, specified that No-Viol glass be used on his PL
signals.  The conical cover glasses and the frosted tips were also Mr.
Rudd's idea.  (The five articles detailing the development of the PL signal
between 1914 and 1922 are a fascinating read; I have placed PDF copies with
the N&W Archives.)

What I have been searching for (unsuccessfully, so far) is a precise date
for the introduction of the PL-2 and PL-3 lines of equipment. The surviving
Union Switch & Signal records do not give dates for first and last
production runs of the PL-1, PL-2 and PL-3 equipment.   Due to the 70 year
copyright law in the United States, Google has not yet released the
digitizations of volumes of Signal Engineer magazine from the 1930s and
1940s, which magazines contain the information I need to solve the dating
problem.   If anyone has a brother-in-law at Google, please help me out...

-- abram burnett,
    turnip farmer

Sent to You from my Telegraph Key
    ... better than AT&T 4G LTE

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