At night, the arm of a semaphore signal becomes invisible so the signal aspect is given by a signal lamp (usually paraffin) in front of which two coloured filter glasses connected to the signal arm are moved:-
'Stop' signals - Display either a RED or GREEN light. 'Distant' signals - Display either a YELLOW or GREEN light.The reliability of the lamp and its ability to continue burning in adverse weather conditions was paramount and considerable ingenuity was devoted to producing suitable designs. Care and cleanliness in the handling and maintenance of signal lamps was essential. 'Lamp Oil' was produced to a high and constant specification to ensure consistent performance. Reports of "Signal Lamp Out when should be lit" were treated with the utmost gravity.
Various designs of signal lamp, produced by different railway supply companies, were in use up to the 1960s and beyond but they had similar features, comprising a substantial sheet steel housing with a hinged lid which was permanently attached to the signal post into which a removable lamp (often called the 'vessel') could be fitted.
'ADLAKE' Lamp Housing, showing 'Bull's Eye' lens, hinging top and chimney (10p coin alongside for scale).
The front face of the housing mounted a large clear glass 'Bull's Eye' lens which focused the light through the coloured filter glasses of the signal arm.
'ADLAKE' Lamp Housing, viewed from above with hinging top opened and removable lamp assembly in position. The fillet in the back left corner of the housing ensures correct alignment of the lamp assembly.
A simple spring clip unlatched the top of the lamp housing, allowing it to hinge open for access to the removable lamp, which simply lifted out using the wire handle provided.
View of lamp removed from lamp housing. The wire carrying handle is in the 'stowed' position.
The large tank forming the base held lamp oil sufficient for 8 days continuous burning with a correctly-trimmed burner. On this pattern, all four sides of the hinging top section protecting the burner were provided with a clear glass.
'ADLAKE' Signal Lamp, opened to show burner and chimney. The locating lugs for the wick adjuster ensure correct alignment of the flame. Screw filler cap for the vessel (with retaining chain) is near front right. The sliding wire clip which latches the hinging top section closed is front right.
The burner has a metal body with a carefully-shaped porcelain insert which helps to produce a fairly broad, flat flame. A flat, woven wick is held in position in the centre of the porcelain insert, with a long 'tail' reaching down into the tank of lamp oil. The wick adjuster is turned to provide the correct length of wick projecting above the burner.
Rear view of this type of lamp housing fitted to an upper quadrant tubular post signal. The signal lamp has a clear lens at the back, the 'back light'. The cast arm at 'six o'clock' with the curved screen is the 'Back Blinder'.
At night, stop signals in advance of the box (that is, leading trains away from the box) could sometimes be seen by the controlling signalman who was able to verify the signal aspect by observation since the red or green indication was facing the signal box. But, in the case of stop signals in the rear of the box, the indication faced away from the signalman. To provide an indication of the aspect displayed at the rear of the signal, signal lamps and the lamp housing deliberately showed a white 'Backlight' at the rear. The lamp itself had a clear glass window and the lamp housing had a small clear glass 'Bull's Eye' lens which focused the light. The exact position of the 'Bull's Eye' left and right was adjustable to optimise the visibility to the signalman. The pivot for the signal arm was provided with a cast arm (at '6 o'clock in the view above) with a curved screen called the 'Back Blinder'. If the signal arm was 'On', the 'Back Blinder' did not obscure the 'Backlight' and the white light was visible to the signalman, confirming that the signal was 'On'. But, if the arm was off (or partially off) the movement of the 'Back Blinder' obscured the 'Backlight'.
Of course, not all signals could be visually checked by the signalman due to position or line curvature. Signal repeating (using electrical techniques) was developed to assist the signalman and I'll describe this in another article.
General view of a signal lamp used by the L.M.S. This was the "ADLAKE" No. 22, made by Lamp Manufacturing & Railway Supplies Ltd.
View from above showing this type of lamp housing fitted to an upper quadrant lattice post signal, with the lamp housing open to show the lighted signal lamp inside. At the top of the picture can be seen the filter glasses producing the appropriate signal aspect.
The picture above shows the red filter glass giving the 'Stop' indication. When the arm is raised to 'Proceed', the other filter glass is moved in front of the lamp. Note that the second filter glass is blue, not green. The signal oil burns with a yellow flame. When the yellow light passes through the blue filter glass, the result is a vivid green 'Proceed' indication.
High-intensity signal lighting
View other sizes.Brereton Sidings starting signal 'off' with fixed distant for Rugeley Trent Valley below. Both signals are electrically lit and have shaped spectacle glasses, rather than the flat filter spectacle glass used with paraffin-lit signals. The disc signal mounted on the right of the post controls movements into the Power Station and has the remains of a 'secret until lit' route indicator mounted above.
Electric illumination of semaphore signals was also used for some applications. The L.M.S. adopted what they called 'High-intensity signal lighting' in a number of places where clearance or sighting limitations made the use of full-size signal arms impractical. The brighter light indications provided compensation for the poorer visibility of the miniature signal arms. The installation shown in the picture above is in conjunction with full-size arms, but on the approach to the Trent Valley Line which has colour-light signals.
My Pictures
British Railway Signalling Equipment
Go to Part 5 - Signal Arm, Slot and Lamp Repeaters.
