LAND AND WATER 
May 8, 1915 
SEARCHLIGHTS 
By "A.M.I.CE." 
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Diagniiu of a Projector throwing a Parallel 
lj«ain of Light. 
THE Voltaic Arc was discovered by Sir Humphrey 
Davy about the year 1808. In that year Davy 
obtained the money to construct a primary 
battery consisting of 2,000 cells with which he 
showed at the Royal Institution the experiment 
that created so much attention in those days. Two pieces of 
charcoal were connected by wires to this powerful battery ; 
they were brought together until they touched, and then 
separated to a distance of about \ inch apart from one another, 
when a flame of dazzhng brightness was obtained between 
the two charcoal pieces. The light so obtained was known 
under the name of the electric arc, because the flame passing 
between the pieces of charcoal was in the form of an arc. 
In a modem arc lamp 
carbon rods are employed 
instead of charcoal. As 
the electric current will 
not jump the gap between 
the ends of the carbon 
rods, it is necessary to 
bring the ends together 
until they touch each 
other, and then separate 
them a little. Technic- 
ally this is known as 
striking the arc. The 
distance between the carbon rods is automatically kept con- 
stant by the mechanism of the arc lamp, which is worked 
by the electric current passing through the lamp. 
As the arc continues to bum the carbons are gradually 
consumed. When continuous current is used (this is nearly 
always employed for searchUghts) the carbon rod which is 
connected to the positive pole of the electric circuit burns 
away faster than the other one, while at the same time they 
assume different shapes. A cavity or crater is formed in the 
end of the positive carbon, while the negative carbon becomes 
slightly pointed. By far the greater part of the light comes 
from the white hot end of the positive carbon. 
Grade searchUghts were first employed for miUtary 
purposes during the American Civil War, while in the Franco- 
German War of 1870 they were used during the siege of Paris, 
when the Germans in vain tried to surprise the forts during 
the night. Similarly the besiegers employed them for studying 
the French positions at night. The French ships operating 
against the German ports also employed searchlights. 
After the Franco-German War, experiments were carried 
out to perfect the searchUght for naval and military purposes, 
and in the Spanish-American, South African and Russo- 
Japanese wars, the importance of searchUghts on board ships, 
in fortresses, and in the field was abundantly demonstrated. 
In the early lamps attempts were made to concentrate 
the Ught emitted by the electric arc lamp by means of glass 
lenses similar to those used in lighthouses. Fresnel was the 
first man to concentrate successfully the beams of an arc 
lamp by means of special lenses. 
A great step in advance was made in 1876 by Colonel 
Mangin, of the French engineers, who introduced the Mangin 
mirror as a reflector. This mirror consists of a silvered glass 
reflector having the inner and outer surfaces ground to curves 
of different radii. In 1886 Schuckert, of Nuremberg, succeeded 
in producing a glass paraboUc mirror ground to mathematical 
accuracy. Mangin's mirror had the same effect as a para- 
bolic surface in producing a paraUel beam of light. The 
disadvantage was, however, the liability to fracture when 
cooUng, owing to the unequal thickness of glass produced 
by the unequal radii. Since the introduction of the paraboUc 
reflector very few changes have been made, and for aU practical 
purposes the ordinary searchlight of to-day is the same as 
twenty-five years ago. The reflector consists of a parabolic 
mirror, made of glass, carefully ground and poUshed to the 
requisite curvature and silvered on the back like an ordinary 
looking-glass. Such a mirror has the mathematical property 
of sending out all light from the focus of the parabola into 
space as a paraUel beam. 
Diverging beams of Ught are used for na^•igating purposes. 
In the Suez Canal a special dispersing lens is employed, giving 
a wide beam of light, having a dark band in the centre, so 
that no direct light is thrown on approaching vessels, while 
at the same time the sides of the canal are illuminated. 
In passing through the air, the elements of the beam 
encounter various impurities such as dirt, carbon, particles 
of moisture, etc., which absorb, reflect and refract the rays 
of light. From this cause the beam is visible, while at the 
same time it faUs off rapidly in intensity. Naturally the 
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Diagram Illustrating the Use of a Lens to 
Obtain a Dispersed lieam. 
effective range of searchlights depends largely on atmospheric 
conditions. 
For tactical purposes searchlights are classified (a) 
moving, ip) fixed. A moving light can be moved horizontaUy 
or verticaUy through an angle only Umited by the construction. 
Such a light can be directed on any target within its field of 
action. A fixed light is kept steadily on one bearing and 
elevation for the illumination of one definite object. A con- 
centrated moving Ught is called a searchUght or searchbeam, 
while a concentrated fixed light is called a " sentry " light or 
beam. Dispersed lights are generally fixed. To increase 
the range two or more Ughts are often employed in such a way 
that the beams illuminate the same area. 
Various types of portable searchUght sets have been 
developed. The power wagon carries the petrol engine 
dynamo and other accessories, while another carriage contains 
the portable searchlight, and sometimes an expanding tower 
on which the projector is mounted. The wagons can, of 
course, be arranged for motor or horse traction. Motor 
lorries have also been constracted to combine the generating 
set and searchUght all in one. 
SearchUghts in the field should be located so as to be able 
to search effectively the approaches of a battle area with a 
view to an early discovery and identification of the enemy. 
They should be weU entrenched, placed if possible on the 
flanks of the position, and be on a higher level than the defend 
ing troops. 
For observation the range of searchUghts on land is from 
2,000 to 3,000 yards, and further for illuminating purposes. 
Moving objects can be 
more easily identified 
than stationary ones. 
Searchlights, when used 
for defence, should not 
be exposed too much, as 
they might give away the 
position and actuaUy 
guide the enemy when 
making a night attack. 
On the other hand, the 
dazzUng Ught is most 
disconcerting to attackers, 
and has the effect of distorting ranges. Cases have been 
known where a range of 8,000 yards was taken as 3,500 yards, 
due to the bUnding effect of a searchlight. For this reason it 
is a difficult matter to damage a searchlight by gunfire. 
In coast defence stations and fortresses, numerous 
searchlights are installed. More powerful lights can be em- 
ployed in such cases than in the field, owing to the possi- 
biUty of obtaining practicaUy an unlimited supply of electrical 
energy from permanent power stations. In the field the 
movements of the projector are, as a rule, controUed by 
hand, but in fixed stations smaU electric motors operate 
the vertical and horizontal movements of the lamp. The 
movements of the searchlights are generaUy controlled from 
directing stations, which are made as inconspicuous as 
possible and placed on a higher level than the Ughts. 
The electric motors, which operate the movements of the 
projector, are controlled from the directing station, which is 
in telephonic communication with the officer responsible for 
the tactical employment of the lights. 
In the fleet, searchlights play a very important role. 
By the aid of them it is possible to repel torpedo and submarine 
boat attacks on dark nights. The blinding beam of Ught, 
well handled by the defenders, increases the difficulties of 
attack enormously. After the beam of light has been turned 
away, it is difficult for the attacker to see again accurately, 
owing to the impression left on the retina of the eye. 
From various accounts it appears that the Zeppelins are 
fitted up with searchlights. These are used probably for 
reconnoitring purposes and locating targets for bomb drop- 
ping. The difficulty of fitting aeroplanes with powerful 
searchUghts is considerable owing to the impracticabiUty of 
generating sufficient electrical energy. 
Quite recently there has been introduced into the Swedish 
Army an oxy-acetylene searchlight. In this light a mixture 
of acetylene and oxygen is burnt which produces a colourless 
flame, having a very high temperature. This flame is allowed 
to play on a small pellet of ceria, which becomes incandescent 
and emits a powerful light. The acetylene is carried in liquid 
form, and the oxygen is compressed in a small steel cylinder. 
The outfit is extremely portable, and sufficient gas is carried to 
keep a Ught going continuously for twenty hours. It seems 
to have a future, especially for portable field searchlights. 
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