CHAPTER IV THE HELIOGRAPH (PAGE 48 OF THE 1905 SIGNALLING HANDBOOK)
The heliograph is a signalling appliance by which the sun's rays can be reflected by means of a mirror or mirrors.
The mirror is actuated by a key. When the key s pressed, the mirror is moved so that the reflected ray is thrown towards the receiving station. When the key is released, the mirror returns to its normal position in which the reflected ray cannot be seen by the receiving station.
Thus by working the key in accordance with the morse code, flashing signals are transmitted to the receiving station.
91. Advantages and disadvantages of the Heliograph
Rapidity. With a well-balanced instrument the labour of sending is no more than that of using a morse key, and therefore a high rate of transmission can be kept up for a long time.
Communication is opened more rapidly than with other instruments as the flash from a heliograph is readily seen.
Can only be used during sunlight, except for very short ranges obtained by using moonlight or artificial light.
To obtain best results much practice is required.
Very careful handling is necessary or the instrument rapidity becomes unserviceable. The mirrors are very easily broken.
92. Use of the duplex mirror.
When the sun and the receiving station arc in opposite directions, it becomes impossible to reflect the light in the required direction from one mirror alone. In this case, a second mirror (called the Duplex) is employed, by which the sun's rays are reflected from the signalling mirror to the duplex mirror, and thence to the receiving station.
The duplex mirror is always employed when the sum of the angles of incidence and reflection is 90 degrees or more
As the light striking the duplex mirror always come from the signalling mirror it is unnecessary to move the duplex mirror in order to keep the light an the receiving station, but the signalling mirror must be moved to keep the light an the centre of the duplex mirror.
93. Use of the dispersing attachment
A dispersing attachment is provided to facilitate communication from shore to ship. A lens increases the dispersion of the beam to 15 degrees and only slightly decreases its intensity.
Consequently, it enables the heliograph to be used for communication from the shore to a bombarding ship under way, swinging at anchor or under conditions when the ordinary heliograph could not be used owing to its very directional qualities.
94. Use of a second Heliograph.
When using the duplex mirror in a high wind, it will be found difficult to read the signals owing to vibration. To overcome this difficulty, the duplex mirror may he removed and the signalling mirror of the second heliograph substituted for it; the latter being on its own stand, which should be placed close to the stand of the signalling heliograph it will be necessary to affix a paper vane over the unsilvered spot of the second heliograph.
95. Range of the He1iograph
The rough distance in miles at which the heliograph can be used, is obtained by multiplying the diameter of the mirror in inches by a factor of 10 miles. This range is governed by the angles at which the rays strike and leave the mirror and the state of the atmosphere.
The total lateral range (divergence) at which the heliograph can be read, is obtained by dividing the distance between the instrument and the receiving station by 107. The lateral range on either side of receiving station will be (approximately) half the total lateral range.
96. The Heliograph.
Description of the instrument
The pattern of heliograph supplied to the Naval Service is the Mark 5 (Patt. 1699). The size of the exposed mirror is 5 inches.
The typical range under normal atmospheric conditions is 70 miles,
The general arrangement is as shown in Figs. 28, 29 and 30.
The tangent screw (E) gives the mirror horizontal movement.
A vertical screw rod (O) gives the inclination of the mirror The inclination is regulated by clamping screw (P) for coarse adjustment, and for final accurate alignment by collar (M).
The heliograph with sight-arm, two sighting rods duplex mirror, two spare mirrors, spare parts and adjusting tool is carried in a leather case provided with shoulder and waist straps, the latter used to prevent jolting when riding.
|A. Stand. Al, brass head.||O. Vertical screw rod|
|B. Anchoring Hook||P. Clamping screw for vertical screw rod|
|C. Sight arm with clamping screw C'||Q. German silver ball attached to claw Q'|
|D. Tangent box with lid below||R. Adjustment screw for German silver ball|
|E. Tangent screw.||S. Base plate.|
|F. Key.||T. U arms, with detachable portion T' and Screws adjusting|
|H. Key bridge.||-|
|I. Key bearings||V. Unsilvered spot at centre of signalling mirror|
|J. Beat regulating screw||-|
|K. Lock nut for beat regulating screw||W. Duplex mirror with frame in U arms (T)|
|L. Swivel joint||X. Sighting vane on duplex mirror|
|M. Collar||Y. Butterfly and pivoting screws|
|N. Vertical socket||Z. Sighting rod with moveable vanes Z'|
97. Setting up Heliograph and stand.
Erect the stand keeping the brass head as level as possible Unscrew the cap. The stand must be firm and rigid, the legs, where possible, being securely fixed in the ground. Holding the stand just below the head, press each leg in turn firmly into the ground, keeping the head of the stand as level as possible, and taking care that the pressure is applied along the direction of each leg, and not vertically so as to cause it to break.
Suspend the heliograph case over the legs of the stand, with the plain side of the case outwards
On hard or rocky ground, where the legs cannot be driven in, stability may be obtained by suspending a weight from the anchoring hook under the head of the stand. The weight should just touch die ground so that it cannot swing.
Take the signalling mirror out of the case and hold it, socket uppermost.
Withdraw the sight arm from the case and place it, rib uppermost over the main socket and tighten the clamping screw.
Invert the whole assembly and screw it onto the brass head (M) of the tripod
Ensure the mirror is clean and free from dust.
Move the instrument till the signalling mirror roughly faces the sun.
98. Aligning the Heliograph
The adjustment of the heliograph must be performed with the greatest accuracy. If the alignment is too high, every time the collar is depressed the light will pass through the receiving station to a point above it, and on the pressure being relaxed the light, during its return to the position of rest, will again pass through the receiving station. Consequently each dash will appear as two dots, and the dots will be broken up into a quivering light and be very difficult to read.
On the other hand if the alignment is too low the receiving station will not see the full light.
First decide whether it will be necessary to use the sighting rod or the duplex mirror, If the angle formed by lines joining die heliograph to the sun and the receiving station respectively is more than 90 degrees, the duplex mirror must be used.
When there is any doubt, consider whether the sun is opening or closing the angle. lf it is opening, the duplex mirror must be used and, if closing die sighting rod. This will obviate a further change after work has commenced.
99. Alignment by using sighting rod.
Move the instrument till the signalling mirror faces midway between the sun and receiving station.
Unclamp sighting arm, place it in die direction of receiving station and then clamp it.
Elevate mirror so that the refection of the receiving station may be readily seen.
Withdraw sighting rod (Z) from case and place it in sight arm, clamping it so that it moves stiffly.
Turn vane (Z') down.
Depress the signalling key and clamp it (see Article 103 (b)).
Turn your back to the sun and look into the mirror.
Move the sighting arm horizontally and vertically as necessary, to bring its cross wires into line with the receiving station and the unsilvered spot.
To Direct the Light
The light is now thrown ort the receiving station by turning the signalling mirror with the slow motion screw. As it is impossible to see whether the reflected light is actually on the receiving station the vane (Z') is brought into play. The vane is turned up and now covers the cross wires on the sighting arm, thus coming into alignment with the receiving station and the mirror.
In the centre of the mirror is a small unsilvered dot, which causes a small black spot to form in the centre of the reflected beam. When this spot is an the centre of the vane, the beam itself will now fall directly onto the receiving station.
NOTE.--in order to direct this spot on to the sighting mark, extend the hand or sheet of paper behind the vane and feel for the reflection of the sun and convey it to the vane as above.
1OO. Aiming Heliograph using the Duplex mirror.
Move the instrument till the signalling mirror approximately faces the sun Unclamp the sight arm and place it in the direction of the sun.
Withdraw the duplex mirror from the case am! place it in the sighting arm parallel to the signalling mirror.
it is now necessary to determine the position of the sight arm.
Stand with your back to the sun.
Imagine a line and its prolongation joining the sun and the signalling mirrors. Then move the sight arm and duplex mirror to that side of the line on which the receive station is situated, keeping the mirrors parallel. It must be borne in mind that the sight arm should be kept as near as possible to this line, but sufficiently far to the side to prevent the duplex mirror intercepting the sun's rays from the signalling mirror.
Clamp the sight arm.
Key should be depressed and clamped (See Article 103 (b)).
Adjust the clamping and butterfly screws (Y and C, Fig. 30) so that the mirror moves stiffly,
Standing with your back to the sun and looking into the signalling mirror, move the head of the apparatus until the reflection of the sight mark on the duplex mirror is hidden by the unsilvered spot in the centre of the signalling mirror.
Keeping the apparatus head still, turn the duplex mirror horizontally and vertically until the reflection of the receiving station, is seen in the signalling mirror and is exactly covered by the reflection of the sighting mark so that the unsilvered spot on the signalling mirror, the reflection of the receiving station and the sighting mark on the duplex mirror are in one straight line.
Tighten the clamping and butterfly screws taking care not to disturb the position of the duplex mirror recheck the aligning and then direct the light.
To direct the Light
The light is directed in the same way as when using the sighting rod, only in this case the spot must fall in the centre of the sighting mark on the duplex mirror.
101. Working under cover
By directing the light from one heliograph onto a second conveniently placed one with its mirror in the proper position, signals may be conveyed to the receiving station without exposing the signalman, the signals being either thrown through embrasure or over the top of a parapet or other cover.
a parapet or other cover. Every time light is reflected its intensity is diminished. and therefore the positions of the heliographs should be chosen so that the light will not be reflected more often than necessary . To prevent loss of light, the heliographs should be set up as close together as possible, and consistent with cover being obtained for the signalman.
The method of setting up anti aligning the heliograph will be made clear by the following example:-
Suppose AB to be the crest line of a parapet. and suppose S to be that of the sun, and D of the distant receiving station -
Position under cover.
Heliograph raised so that it can align onto distant receiving station.
First consider the positions of S and D and decide on the approx positions for the heliographs as at P and Q, P being the signalling heliograph under cover and Q the secondary heliograph exposed.
Align the heliograph at Q on the receiving station using the sighting rod,
Align the heliograph at P very accurately on the centre of the one at Q.
Reflect the sun's rays from P on to Q, and keep the light exposed.
Move to Q and find these rays after reflection from the secondary heliograph: turn the mirror of the latter so that the shadow spot strikes the sighting mark.
Under certain conditions the light from the sun will strike this heliograph directly; in these cases care must be taken that it is the light as reflected from P and not that from the sun which is made to cover the sighting vane.
Return to P, regulate the beat and proceed as usual
By placing the eye level with the mirror at P and looking into the mirror at Q, signals made by the receiving station may be seen or preferably the duplex mirror may be used for this purpose, by placing it in the sight arm of the exposed heliograph, at a convenient angle, the sighting rod being no longer required.
102. Working with dispersing lens
To use the dispersing lens with the heliograph the following fittings are supplied :-
- Dispersing Lens Arm.
- Dispersing Lens Mount.
These are stowed in a separate leather carrying case which is provided with a shoulder strap.
To set up the Heliograph using the Dispersing Lens Patt No 1069 -
Erect the stand, keeping the brass head as level as possible. Unscrew the cap. The stand must be firm and rigid, the legs. where possible, being securely fixed in the ground. Holding the stand just below the head press each leg in turn firmly into the ground, keeping the head of the stand as level as possible, and taking re that the pressure is applied along the direction of each leg and not vertically so as to cause it to break.
Suspend the heliograph case over the legs of the stand, with the plain side of the case outwards.
Take the signalling mirror out of the case and hold it, socket uppermost.
Withdraw the sight arm from the case and place it rib uppermost, over the main socket and tighten the clamping screw.
Screw the special adaptor into the main socket.
Place the dispersing lens arm, rib uppermost on to the bottom of the special adaptor, and tighten the clamping screw.
No invert the whole and screw on to the tripod.
Move the instrument till the signalling mirror roughly faces the sun.
Method of Attaching Dispersing Lens and Mount to Dispersing Lens Arm ---
Figs. 32 and 33 show the arrangement of the dispersing lens arm.
This consist of a bush (shown in half-section in Fig 33) which is normally free to rotate about a vertical axis, in a. split bearing at the end of the dispersing lens arm. The bush can locked in any desired position by tightening the thumbscrew. The bore of the bush is square and at the lower end of the bush a T slot serves to retain a knurled nut.
The dispersing lens is mounted in a circular frame having a spill attached The upper part of the spill is square to fit the bore of the bush and the lower part is threaded to engage the nut.
Assembly of the dispersing lens and mount on its arm effected by first slacking back the thumbscrew, thrusting the spill of the mount into the bush until the threaded portion engages the adjusting nut, and then rotating nut.