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Spotlight to DIY Searchlight






 with Illustrated Carbon Arc Notes






by Larry Brian Radka




“Spotlight to Searchlight,” you say.  “Isn’t a spotlight the same as a searchlight?”





Yes, and No!  Their official definitions have always been somewhat vague and changing.  A spotlight is now usually thought of as a small light generating device that sends out a beam into a concentrated spot, a little larger than today's common flashlight, even though this small light was once even called the same name as its biggest brother—the searchlight.  The copy below of a page from a 1915 electrical equipment catalog attests to the fact:



However, thereafter until now a searchlight has been more often thought of as a much larger apparatus than either a flashlight or spotlight, and it is expected to send out a more intense and brilliant shaft of light, with generally the same type of light pattern.

Modern searchlight beams of light, unlike those of a common spotlights or 1915 portable tungsten-filament searchlights or flashlights, travel a much greater distance—even sometimes in excess of distances covered by some of the monster carbon arc anti-aircraft searchlights of World War II.



Though all may be used for the same purpose, namely to spot or search out something in darkness, raising a spotlight from its light level to the much higher light intensity of what now is accepted as a searchlight is usually no easy task.


However, Larry Brian Radka, in his DIY (Do-it-yourself) project, managed to do so when he created a brilliant carbon arc searchlight—affectionately named “Little Blue”—out of an old spotlight that hardly produced much light at all,



as the photographs above and below demonstrate.



“Little Blue,” originally a very simply designed spotlight, was built from an early 1900’s Crouse Hinds filament type 200 watt WWI-era spotlight-issue, with its original military paint illustrated here.




I purchased this working antique recently on Ebay for $150.00, which sounds like a lot, but old spotlights often bring a much heftier price.  I measured the temperature of the spotlight inside with its front window closed, and it measured 200 degrees Fahrenheit and stabilized after operating for a few minutes.  That's when I realized I would probably have to install a fan and vent to allow it to use a second sun which could generate temperatures up to thousands of degrees inside the spotlight case.



Note the homemade adjustments I installed in it for its now high intensity carbons—to keep the Direct Current arc centered on the focal point of its 12-inch metal parabolic mirror that replaced its old concave aluminum reflector.  A silver-backed glass mirror seems to have a higher reflective power, but can easily break from the intense heat generated by the arc.  I proved this on my 8-inch parabolic mirror during my self-taught course in the “School of Hard Knocks.”  Another reason for using a metal mirror is to prevent breakage from an abrupt shock.  This is why the military has preferred metal metals.  Note the small round arc light window in the center of the mirror behind the carbons.



I installed this to protect my eyes from its harmful ultraviolet rays while adjusting its blazing carbons.



The peephole glass, viewed through the rear to get a good view for adjusting the tilted positive carbon for more exposure to the focal point of the mirror, was taken from one of my automatic carbon arc microscope-projector lamp houses.

Little Blue's separate manual adjustments for positive and negative carbons is shown protruding from the bottom of searchlight in the photograph below.  They may be ganged, and with the proper sizing (the positive burns up faster) I may decide to using a timing belt to drive them with one of my very slow General Electric 12 VDC searchlight motors, purchased on Ebay for a mere $5 a piece.
Note the 24-volt DC squirrel cage fan that I installed in the back of this searchlight.  Its high-volume air flows around the back of the mirror throughout the case and up through the homemade chimney at the top. This keeps the mirror cool and quite free from the burnt white carbon that likes to latch onto its face.   Nevertheless, the fan's air flow does not prevent the transparent glass cover to get very hot, from its resistance to the intense beam of light blazing through the front of the searchlight.

The DC arc in this little searchlight creates an asymmetric operation and different individual carbon conditions.

A crater usually forms in the tip of the positive electrode, as is illustrated above, and it burns up more rapidly than the negative carbon which is often rounded out into a ball-like shape at the end.

The two often leave a remarkable display similar to the shapes in ancient illustration above, found in the 2000-year-old Temple of Hathor at Denderah, Egypt.  Note the flames rising from the positive crater and direction of the current flow in the arc running from positive to negative.

 Apparently the ancient Egyptian priests, like modern electrical engineers,

also believed electric current flowed from positive to negative poles when they built their carbon arc searchlights displayed above and below,


on the walls of crypts in the Temple of Hathor at Denderah, Egypt.

In 1904, J. A. Fleming discovered the opposite was true.  See The Electric Mirror on the Pharos Lighthouse and Other Ancient Lightingfor many more illustrations and details.
I should also point out here also that the positive electrode nearly always emits more light than the negative–just like the flames in Denderah display above indicates.  In carbon arc searchlights and projection systems, a DC arc is usually used so that most of the light is emitted from only one spot, the crater in the tip of the positive electrode.
The tip of the positive carbon is then precisely adjusted to blaze away in the focal point of a searchlight's parabolic mirror so most of its light is reflected in parallel int0 a sharply focused beam that may reach out several miles, like those monster 60-inch searchlights, used by Allied and Axis powers alike during World War II.  Those brilliant "second suns" or "electric mirrors" allowed flak crews to see bombers high in the sky, miles away.

Nevertheless, above we see my largest homemade power supply—using the case of an old General Radio signal generator case, purchased on Ebay recently for $20.00.  The main features inside are a transformer to reduce the 240-volt AC from my clothes-dryer jack to 120 volts, a high power heat-sunk bridge rectifier, appropriate meters and adjustments to monitor and control conditions, and 50 and 80 amp inductors (purchased also on Ebay) to shift the voltage out of phase with the current.  The result is (unlike many cheap arc welders having a fan but still a short duty cycle) a blazing arc allowing my power supply (with no fan) to run cool as a cucumber, even with a long, continuous 38 amps supplied to the arc.  Ohm's power law applies to pure DC (this DC is unfiltered) and in-phase AC voltage and current.  This issue involves the difference between real and apparent power, a technical subject that I won't get into here.

However, for those of you who have electrified minds, I will include here a handy picture above and a simple schematic below of a much smaller DC and AC carbon arc power supply that I built on similar principals a few months ago for my carbon arc magic lanterns.

This smaller 120 VAC input unit also runs cool as a cucumber but supplies less current.


I can adjust the input voltage (and especially current) to the power supply with the variable transformer pictured above.  I purchased it on Ebay for $17.00 recently, but it cost about $45.00 in shipping.  These old variable transformers normally sell for upwards of $1000.00 now, so I consider this a great deal.


This label, if you can read it, gives you a general idea of its technical specifications.  I originally did not use this external variable transformer (with my attached meters pictured below) but a 230 VAC alternistor (an improved triac) with fuses and variable potentiometer from a piece of factory machinery (purchased also on Ebay) in my big power supply instead.

However, when I installed the unit inside the primary winding of the 5 Kilowatt 250 VAC to 125 Vac step-down transformer inside the original homemade power supply pictured above, I didn't want to be bothered with replacing any of the fuses originally installed in unit, so I replaced them with 20 amp circuit breakers instead.

That was a big mistake indeed, especially since I only had one and also had failed to properly ground the secondary of its transformer to the case and arc light.  Then the negative carbon fell out of its holder while blazing away and the metal fan enclosure mounted on the back of the searchlight cowl arced over to the power supply case and shorted out the alternistor.  Needless to say, it almost scared the you-know-what out of me.  A little research reminded me that fast blow fuses burn up much faster than circuit breakers kick.  Had I left the original fuses in the factory arrangement with the sensitive alternistor instead of replacing them with circuit breakers, I probably would not have to have bothered with bringing out the big gun above.


That was a big mistake indeed, especially since I only had one and also had failed to properly ground the secondary of its transformer to the case and arc light.  Then the negative carbon fell out of its holder while blazing away and the metal fan enclosure mounted on the back of the searchlight cowl arced over to the power supply case and shorted out the alternistor.  Needless to say, it almost scared the you-know-what out of me.  A little research reminded me that fast blow fuses burn up much faster than circuit breakers kick.  Had I left the original fuses in the factory arrangement with the sensitive alternistor instead of replacing them with circuit breakers, I probably would not have to have bothered with bringing out the big gun above.


Neverthess, the brute electrical force of this variable transformer will insure that I will have many more reliable searchlight operations ahead, as my neighborhood bathes in the spotlight of my new searchlight.


Illustrated Carbon Arc Notes


AC carbon arc searchlights may also be built from old lighting devices.  Above we see a good example in a rare William Brinkmeyer Virginia Sun Ray Lamp that I recently purchased on Ebay for $50.00—an antique that would have sold then for the equivalent of thousands of dollars in today’s devalued American currency.


This beautiful chrome carbon arc tanning lamp was quite easy to change into an 120 VAC carbon arc searchlight in another of my DIY (Do-It-Yourself) projects.  The concave inside back of its lamp case was originally used to cast its artificial sunshine forward onto a solar bather, and the intensity and continuity of its blazing carbon arc light was maintained by the variable adjustment on the side of its case.  I merely installed one of my 12-inch metallic parabolic mirrors inside, adjusted its carbons to burn near its focal point, and voilà—a searchlight!


Note the timing pulley I installed in place of its chrome adjustment on the side of its case, so I could maintain the distance between its burning carbons for continuous operation by attaching a short timing belt driven by one of my small GE searchlight motors.


Above we see a close-up of the cage with the ribbon type wire power resistor that is wrapped on porcelain insulators up and down and all around the inside of the wire wound enclosure. This old voltage dropping and current limiting resistor has probably now survived for a hundred years, still in good condition.


Most AC (Alternating Current) series resistors found today with carbon arc lights, especially in magic lanterns, are useless because their resistive wire has rusted and their ceramic cores have disintegrated from old age.


Once the two carbons of an arc light are touched together to start or “strike the arc,” the AC circuit momentarily sees almost a short, so a current-limiting resistor or resistors (usually totaling 6 to 15 ohms for a 120 VAC circuit) are needed to limit the current surge.  Thereafter, when the carbons are separated for a short distance, the current continues for some minutes to arc between the carbons to provide the light.  However, unlike my homemade carbon arc power supplies illustrated above, this is a very inefficient means of powering any arc light, although they are easily manufactured.


I have made them from large electric heaters and even little fan-cooled dryers.


But all of these waste a lot of power in the form of heat while supplying current to arc lights.  Furthermore, the longer the arc is maintained, the greater the consumption of the carbons and the greater becomes the voltage drop and the less becomes the current between the carbon electrodes,


until the light finally goes out—unless the carbons are continuously kept close to each other.  Note that both carbons above are equally hollowed out from the AC arc that once generated the light that blazed between them.  However, it is noteworthy to point out here that most of the light from any AC arc lamp is emitted from its carbons (neither easily centered on a parabolic mirror's focal point) and not the space between them.


And even that light is less than one third the available light normally emitted from a DC arc lamp (whose single positive hot spot can be easily focused) unless special AC-type carbons are used.  Even then, the light from their chemical flame, springing on and off 60 times a second between their carbons) might only increase the available light by 30 per cent.  When I first fired this AC lamp up with high intensity DC-type carbons like those above, the beam was fairly dim because neither carbon's hollowed-out hotspot was perfectly centered on the parabolic mirror's focal point and the arc produced comparatively little light.  Therefore, I chose to install white flame photographic carbons instead, and their smoky, but long, brilliant white flash (caused by special cored-chemicals) in the arc between the carbons provided much more light to the reflector's focal point.


I hope these brief notes and the pictures above and below give you a general idea of what is electrically going on here with this carbon arc searchlight and hope to do a Web page some day that discusses these technicalities in more detail.  Note, however, the interesting spread-out shape of the convex image (opposite of the concave one inside) reflected from the back of its chrome case.


Here we see an old photograph of a magnified image of a pretty young lady taken inside the focal point of a big parabolic searchight mirror much larger than the one inside my little searchlight, and we can see that it could easily be used for a telescope like the ancients once used.  When standing further back, past the focal point of a concave mirror, where the light beams cross over, we would see an upside down image as illustrated by the cameraman reflected in the mirror of the airport beacon below.  Nevertheless, this would make little difference when viewing magnified stars.  In fact, modern astronomers have often removed the eyepiece from their reflective telescopes to use just its large mirror's highly magnified images to photograph the heavens.  There is more in the The Electric Mirror . . . .


Nevertheless, getting back on track now, I should mention that the power meter illustrated below is inside the little wooden case beside the “power measurement” photograph above this digression.


Here we see a photograph of the actual voltage and power measurements of this AC carbon arc searchlight, with its current and voltage nearly in phase.  This means (excluding the resistance of its carbons and power cable) that its series wire-wound resistor in its screened enclosure was limiting the searchlight's current with about 6 ohms of resistance at the time, considering the extra impedance in the arc itself at the time.

I hope you have enjoyed these "Illustrated Carbon Arc Notes" on my DIY AC carbon arc searchlight.

The animation above shows a serpent in the center of its light beamlike the snake below, in the rounded ray on the wall in the ancient Temple of Hathor at Denderah (Dendera) Egypt.


Researching ancient literature, studying ancient artifacts, and playing with carbon arc searchlights have finally let the ancient Egyptian electrical serpent slither forth into modern view!


For anybody interested in experimenting with carbon arc lights, I will pass this email, written off the top of my head, to a fellow in Australia who shares an interest in this old, and almost forgotten, technology:




I find that the more current you run, the larger the rods should be. For AC running at

120VAC on its carbon arc magic lanterns, Bausch & Lomb recommended I think around

7 mm rods for 5 amps of current on AC.  However, searchlights require much more

current and larger rods.  My Little Blue runs best at about 100 volts AC and 20 amperes

for maximum brightness, using 10 or 11mm rods.  Beyond that, I have increased the

current up to 45 amps (when my circuit breakers blew on the power supply) and the

brightness didn't seem to increase that much at all.  (You can get a cheap camera light

meter if you want to get a better idea of brightness than you can get with the human eye.)


Get that National Carbon Company (little black book) on projector carbons sometimes,

and you can find out much more about carbons there.  I saw one advertised on Ebay a few

days ago.


I like to stay away from copper coated carbons (last longer) and use plain cored carbons. 

The carbons should be the same size and type for AC since they burn up equally.  I used

copper coated carbons on my little 8 inch glass projector parabolic mirror and the hot

metal flying off the copper coated carbons pitted the glass mirror.  (I learn a lot the hard

way.) Projectors use them sometimes, but one has to be careful not to get the arc too

close to the mirror or the problem will arise.


Good luck with your three-phase welder.


Also, I would like to point out that using a fan with AC for cooling the light will tend to

make the arc wander in and out of the focal point of parabolic mirrors.  I don't have that

trouble with DC, since probably 90% of the light comes from the hot spot on the positive

carbon in the focal point of the mirror and doesn't wander except when it burns up too

much carbon and the loss is not compensated for (manually in my case so far).


I hope this helps.




P.S.  Also, keep in mind, the larger the rod, the less resistance there is, and even a half

ohm is significant when running a lot of current at low voltage.  The wires  or cables

should also be as large as possible to reduce the resistance and loss of power that gets to

the light.  You probably already know this, but I thought I would mention it just in case

you didn't.
P.S.  Peter, you are much better off price wise to get your carbon rods off Ebay.  I have

picked up boxes of 50 for 5 or 10 dollars, versus 100 or more dollars from a distributor. 

The carbons are almost always still good after a hundred years.  The only real problem

that can occur with them that I have noticed is that if they get wet or have been stored in

a moist place, but if this should happen (very seldom), they can be dried out and work




Note:  The copper wrapping may easily be removed from metal coated carbons with a box cutter and pliars (see the picture above) to prevent their hot metal from damaging a searchlight or spotlight mirror.  However, the carbons will then burn faster without their metal wrapping.


See The Electric Mirror on the Pharos Lighthouse and Other Ancient Lighting for more on carbon arc searchlights.







This page was last modified on Monday, February 19, 2018