Introduction: Tesla's Candlestick: Wireless Electricity

Tesla coil Instructables are not uncommon--I've written one myself--but in the following Instructable I'd like to not only describe building a simple upright coil, but also suggest some easy ways to assemble Tesla system components. Sometimes the parts of a Tesla coil system are harder to get than the coil itself. Then, at the end, I will demonstrate (in a small way) how Tesla coils can transmit electrical power through the air without wires.

The essence of this Instructable is simplicity rather than brute power or maximum efficiency. Because my coils are small, indoor models, I prefer to follow a modular method of building them. Most Tesla coils are unitized--everything packaged together in a single unit, just plug and play. I like to build in modular fashion so I can switch components around, try different coils, capacitors, or power supplies. Consequently my designs are clipped together with alligator clips and length of high voltage wire. If you
prefer the unitized approach, you can certainly adapt this design that way.

This Tesla coil system consists of these parts:

A high voltage (7,500 volts, 30 milliamps) AC transformer

A pressurized spark gap

A capacitor array

The coil itself, consisting of a primary and secondary coil on some kind of supporting

A "top load," or terminal

Wire to connect everything

Optional components you will want include a variable transformer (variac), and a small
hose-type vacuum cleaner.

Fluorescent light tubes, with stands

Each of these topics will covered on a separate page.

Step 1: Power Supplies

This may be the most expensive part of your Tesla coil system. You will need an AC transformer of at least 3,000 volts at 30 milliamps to get the spark gap to fire, and more is better--within reason. Once you exceed 15,000 volts at 60 mA, you're leaving the indoor, tabletop arena and graduating to garage maniac status. This system is designed around a 7,500 volt, 30 mA neon sign transformer.

Until a few years ago, most neon signs were powered by heavy transformers potted in insulating tar. Nowadays concerns for safety (and liability) have made the old heavyweight NST obsolete. Modern neon power supplies are light, solid state units with Ground Fault Interrupt (GFI) protection. This is all very well if you're trying to light up a Michelob sign, but GFI power supplies won't work in a Tesla coil system. GFI is designed to stop just the sort of continual sparking a Tesla coil requires, so avoid modern neon power supplies. If you can't tell by the label if a transformer has GFI, try this rule of thumb: if the unit is small and light, it has GFI and won't work on your Tesla coil. If it's heavy as lead and almost as big as a cinder block, it will.

Other power supplies can be used. Oil burner ignition transformers (OBITS) can often be had cheaply. They almost always come rated at 10,000 volts at 23 mA. As with neon sign transformers, the newest OBITs now come with solid state circuitry and GFI. Pay close attention to make sure you don't get stuck with a useless GFI model.

Some coilers use microwave oven transformers. My advice is, DON'T. MOTs, as they are called, are rude, crude, and develop lethal amounts of current. If you don't know what you're doing, you can easily harm yourself or others. Until you've learned a lot more about electrical engineering, leave MOTs alone.

Car ignition coils are sometimes used to power Tesla coils. Ignition coils have to be driven by some kind of circuitry, either a 555 timer and associated parts, or even by something as simple as a household dimmer switch. This can be a fun project on its own.

Tesla coils can be powered by DC, but that's a different topic not covered here.

For best and simplest results, get your hands on a neon sign transformer (NST). All the ones I own I got off eBay pretty cheaply. They're heavy and can be expensive to ship, so if your budget is really tight, try a local sign shop. Many businesses are still switching over to electronic GFI power supplies and discarding their old NSTs. You might get a bargain if you offer to take an old-style NST off their hands.

Many NSTs come without power cords. When I salvage parts from old microwave ovens, one thing I always keep is the sturdy, three-wire oven power cord. These are easily adapted to NSTs.

One thing to remember about using NSTs: never move or lift an NST by the ceramic terminals! They're brittle, and they can crack or snap right off, then you've got nothing but a doorstop. Always lift NSTs by the case. One thing you can do to improve the portability of an NST is mount it on a base. I put my 7,500 volt Allanson NST on a shellacked pine plaque, fitted with four rubber appliance 'feet.' This insulates the NST from the tabletop, and gives me a handy place to grab and lift it.

It happens my Allanson unit has a pull-chain on-off switch. This is unusual. Most NSTs I've seen do not have on-off switches. Once you have grafted on a power cord, the best way to handle the NST is through a variac (a variable transformer), or an isolation transformer (see "Variacs and Vacuum Cleaners"). Failing those, you may have to use a wall socket controlled by a wall switch, or else put together a switch using a household wall switch mounted in a utility box.

Step 2: Pressurized Spark Gap

Pressurized Spark Gap

The spark gap is a primitive form of high speed, high voltage switch. It turns the power flow to the capacitors on-off-on-off 60 times a second (assuming you have a gap with two terminals using normal USA household current). This causes the capacitors to build, then dump a heavy charge into the primary coil. The quick pulse of the primary induces high frequency generation in the secondary coil, resulting in corona and arcing from the Tesla coil's top terminal.

A spark gap can be made of ordinary hardware bolts and brackets, and it will work, though not too well. The air between the electrodes rapidly gets hot and breaks down, spoiling the rapid on-off sequence essential to the coil's operation. Two common ways to fix this problem include having a multi-gap spark gap (three or more spaced electrodes, dividing the spark between) or cooling and quenching the spark gap with moving air. The latter method leads us to the pressurized spark gap. I learned this particular design from Stephen Hobley's website:

To make one you will need T-shaped electrical junction box, some short lengths of copper tubing, and a piece of PVC pipe. For a small tabletop coil a 1/2 inch junction will do. These are sold in hardware store and home centers. They're used to join together home wiring from three lines.

Using a 1/2 inch sized intersection, cut two pieces of 3/4 inch (outside diameter) diameter copper tubing about three inches long. Insert one of these in the each end of the long axis of the intersection. They should a snug friction fit. If they are loose, wrap the pipe with a few laters of plumber's teflon tape. Don't let the tubes touch each other in the center. Keep them about 1/16th of an inch apart to start with.

Cut a three inch long piece of 1/2 inch PVC tubing. Push this into the center section of the T. This is your air inlet/exit.

When the Tesla coil array is assembled, leads from the power supply will be attached to the outside ends of the copper tubes. Wires leading to the capacitor array and primary coil will also be clipped to the copper tubes. If you are building a unitized coil, you can drill a small hole in each copper pipe and install screw terminals.

The spark gap will work fine as-is, but for higher output from the coil you must hook up a small vacuum cleaner to the air inlet. Slip the end of a hose type vacuum cleaner over the inlet. If the fit is too lose, wrap electrical tape or a thick rubber band around the inlet until you get a good fit with the hose.

It doesn't matter if the vacuum cleaner blows air into the gap or sucks air in through the copper tubes. Either way the spark gap is cooled. The firing rate of the spark gate will increase, greatly enhancing the output of the Tesla Coil.

Step 3: The Capacitor Array

The Capacitor Array

If you imagine electricity like flowing water, capacitors are like buckets which fill to the brim before tipping over and spilling their contents all at once. In the Tesla circuit, the capacitor builds a charge until the spark gaps fires, then the power is dumped to the primary coil.

Early coil builders used batteries of Leyden jars, an early form of capacitor. In his famous 1899 Colorado Springs experiments, Nikola Tesla used large water bottles filled with brine and sitting in metal tanks as high voltage capacitors. High voltage capacitors are manufactured today, but a new coiler may want to make his own. One popular type is the Bucket Cap, created and popularized by The Geek Group. A bucket cap consists of a dozen long neck beer bottles filled with salt water, housed in a five gallon utility bucket. The bucket cap is too large for this tabletop Tesla coil, however.

Another possibility is my design for a Leyden Jar battery made of aluminum cans and plastic drink mix cylinders. Or you can just use modern, factory-made capacitors.

If you you're going to use manufactured caps, you have to pay attention to the type of capacitor and its voltage rating. Electrolytic caps look like little metal cans or cylinders; they are not suitable for Tesla coils. They will overheat and may even explode (it happened to me in another context).  Generally you want a high voltage capacitor that's also suitable for use with fast pulses of power. There are fancy and expensive caps that do this. For hobbyists and amateurs, high voltage rated film caps or ceramic doorknob caps are probably the best choice.

As for voltage rating, you'll need to use caps whose DC rating is at least double the AC voltage input. In other words, using my 7,500 volt NST, you'll want capacitors rated at 15,000 volts or better. More is better. One way to achieve both a high enough voltage rating and get the capacitance you need is to build a "multi-mini capacitor," or MMC. This means assembling an array of small caps whose combined voltage rating and capacitance meet the needs of your system. For the system described here, I wanted 20,000 volts and 2 nano-farads (2000 pico-farads). These parameters are based on my using a 7,500 volt NST and the system's capacitance requirements.

How do you know how much capacitance to use? Experience first told me that most tabletop Tesla systems require capacitance in the low nano-farad range. I then used a program called TeslaMap to pinpoint my needs. TeslaMap is a spreadsheet style program. Your enter important data on your system--size of the primary and secondary coils, wire gauges, etc. and the program calculates your capacitance needs.

For this Instructable I used four Sprague doorknob caps, each rated at 530 pico-farads at 20,000 volts. ("Doorknob," by the way, is merely a descriptive term. The capacitors look like squarish ceramic doorknobs.) Four caps in parallel gave me 1.97 nano-farads, which worked well with my small coil. The MMC is made simply by screwing 6/32 brass machine screws through strips of copper coated pipe strapping and into the screw terminals of the caps. 

Step 4: The Coils and the Candlestick Frame

A Tesla coil consists two coils of copper wire. The primary coil is usually made from four to ten turns of fairly large gauge wire. On bigger coils it is common to use copper tubing or copper strip. The secondary coil is made from finer wire--in the case of this coil, 24 gauge magnet wire. Magnet wire is made for winding electromagnets in motors. It's insulated with a thin coating of enamel or plastic. The gauge of wire determines how many turns you'll get per inch of coil.

For both coils you'll need a form to wrap the wire around. Secondaries are commonly wound on cardboard, PVC, or acrylic tubing. For a small coil cardboard is fine. It's lighter than PVC and easier to get than acrylic. These days primaries are often wound as flat coils, but in this Instructable I am using a 3.5 inch PVC pipe coupling to make a helical primary coil. The coupling is also 3.5 inches tall.

Since the primary is smaller, let's make it first. Be sure your PVC coil form is clean and dry. Around the center of the coupling wrap a single layer of double-sided carpet tape. This stuff is incredibly sticky, so take care and get it on straight. For wiring I used 10 gauge house wire with heavy THHN (plastic-nylon) insulation. TeslaMap calculated 6 turns would be optimal with my 2 nF capacitors. Given the circumference of the coupling, I figured I need about 6 feet of wire, plus a little more for the leads, so I got 6.5 feet of 10 gauge THHN wire. I wrapped this around the carpet tape, which helped hold it in place until I could further tack the wire down with a hot glue gun. I bent the leads out at about 90 degrees and attached No. 10 spade connectors to the ends. To help keep the leads from pulling loose I looped a few strands of a heavy rubber band around them.

For the secondary I chose a heavy duty cardboard paper towel tube, 12 inches long and 1.75 inches in diameter. Cardboard must be sealed and insulated before you wind the wire on. I gave the tube two coats of shellac. Allow it to dry thoroughly between coats.

About 1/2 an inch from one end of the dry, shellacked tube drill two little holes about 1/4 inch apart and parallel to the end of the tube. Thread the end of your 24 gauge magnet wire into, then back of the hole, leaving six to eight inches extra as your lead. Threading the secondary wire into the tube is considered a no-no by modern coilers, but old electrical books describe this method of anchoring the secondary. All you need to do is cover the small bit of wire inside the tube with a square or two of electric tape, and everything will be fine. I've used this method on several small coils, and never had a problem with it.

Begin winding your wire. For a secondary this small, fancy jigs or winding frames seem unnecessary. Keep the wire taut (not too tight!) and wind it smoothly, row against row until you reach 1/2 an inch from the other end of the tube. Repeat the procedure of drilling two small holes. Cut off the wire, leaving a generous amount of lead, and thread it in and out of the holes. Tape the inside loop as before. On a 12 inch long tube, leaving 1/2 an inch of free space on either end, you end up with 11 inches of wire winding. According to TeslaMap, this gives you 515.9 turns, or 236.2 feet of wire. 

The next step is to seal the secondary coil. Paint or spray on a coat of polyurethane or shellac. This both improves the insulation on the wire and helps bind the coil in place. Once dry, give it another coat. Depending on how anal you are, you can give many more coats of varnish, but two is really sufficient. Some coilers encase their secondaries in thick layers of clear coating. While this undoubtedly improves insulation, etc., and looks impressive, it isn't really necessary for such a small coil. As a final anchor, wrap a layer or two of electrical tape around each end of the secondary winding.

Now we need a frame to mount the two coils. You can use wood, well varnished, but not metal. I use PVC plumbing pipe. It's cheap and easy to work, and PVC is a good insulator.

Here are the parts for my "candlestick" coil frame:

about 24 inches of 1/2 inch PVC plumbing pipe (Schedule 40, white)

1 1/2 inch "X" joint

1 1/2 inch "T" joint

1 1/2 inch 90 degree elbow joint

4 1/2 inch end caps

1 1/2 inch pipe plug

1 1 inch long, 1/4 size nylon bolt, with nut

1 1 inch long, 1/4 size nylon bolt with 2 nuts

1 nominally "2 inch" plastic test cap. These are knockout caps made to fit 2 inch PVC pipes. My example fits the 1.75 inch paper towel tube perfectly. You may have to test and pre-fit pieces to find the best fit.

Cut three lengths of 1/2 inch tubing, about 4 inches long. Fit these into the X joint. Put an end cap on each of these. Drill a 1/4 hole in the exact center of the test cap. Also drill a 1/4 hole in the top center of the X joint. Don't drill all the way through the X joint, just the upper layer. Insert a 1 inch nylon bolt down through the bottom of the test cap. Put a nut on the other side of the cap and tighten finger tight. Insert the end of the nylon bolt into the 1/4 hole drilled in the X joint. (The next part is a little fiddly). Put a 1/4 nylon nut on your finger tip. Slip this in the open junction of the X joint and catch the end of the nylon nut. Twist the cap on until the interior nut is tight. 

Cut a 2 inch length of PVC tubing. Push it into the X joint's open junction. Attach the T joint with the 90 degree opening pointing up. Cut a 1 inch piece tubing and push that into the other end of the T. (See photos). Drill a small (1/16th) hole in the center of a pipe cap; also drill a 1/16th hole in the X joint above the "leg" that has the T joint on it. 

Put a bead of silicone sealer on the inside rim of the test cap. Take the secondary coil and feed the lead wire at the bottom end through the 1/16th hole in the X joint. Work the out below the T joint end. Thread the wire through the 1/16th hole in the end cap and press the cap to the end of the open tubing. Press the secondary onto the test cap. Let the silicone dry. 

The next measures are subject to trial and error on your model.

Cut a piece of 1/2 inch PVC about two inches long. Push this into the upright joint of the T. Drill a 1/4 hole in the center of the pipe plug. Fit the plug into the 90 degree elbow, and put the elbow onto the upright tubing from the T joint. 

Slip the primary coil over the secondary. It's best if the leads from the primary wire face away from the T joint.

Let the primary rest on the four legs of the X stand. Adjust the elbow so that its upper edge is flush with the top edge of the primary coil form. Mark a spot on the primary form and drill a 1/4 hole. Remove the primary from the Tesla coil. Insert a 1/4 nylon bolt through the hole from inside the primary form. Pull out the pipe plug from the elbow, fit it to the bolt and attach the last 1/4 nylon nut. Tighten. Press the plug back into the elbow, bring the primary assembly down over the secondary coil, and press the open end of the elbow to the T joint connection. Adjust the tubing and joints to get the primary centered around the secondary coil.

The coils and frame are done.

Step 5: The Top Terminal

The top terminal of a Tesla coil is a simple thing, but what it does is complex. It's a terminal and also an antenna from which the power of the Tesla coil radiates. Modern high-power coilers universally aspire to the use of toroids (metal donuts) as the optimum shape. Basically the toroid shape serves to maximize spark length. Other terminal shapes are possible. Small coils can be built with telescoping metal antennas, such as are found on portable radios or TV rabbit ears. By telescoping the antenna in or out you can fine tune the output of your coil.

A hundred years ago amateur coil builders mostly used metal balls or spheres. Good conductors make good terminals--copper, brass, aluminum are preferred but not required. Metal balls are easy to find (toroids have to be made, or specially bought, and that's expensive). Any round metal object will work: doorknobs, lamp decorations, etc. The rule of thumb to remember is that the sphere should be about equal in diameter to the secondary coil.

My top terminal came from a paper towel holder (it seemed fitting, considering I was using a paper towel tube for my secondary). It looks like stainless steel but further tests suggest it's matte-finished nickel plated steel. It has an M8 metric bolt already mounted, so it makes a handy top load.

It's a good idea to have some kind of spark shield at the top of the Tesla coil's secondary. That's where voltage is highest, and arcs from the top to lower turns of the coil are bad. If you damage the secondary's insulation, your coil is ruined. I found a 2 inch PVC pipe cap. It's a slip-fit over the secondary. I bored a hole in the center to accept the M8 bolt. The last inch or so of the secondary lead wire I stripped of enamel insulation by pulling it through a folded piece of sandpaper. Inside the cap I attached the bared lead to the M8 bolt with a strip of aluminum HVAC tape. You can just twist the wire on, or crimp on a ring connector, etc. 

It's also a good idea to isolate the top connection from the core of the secondary. You don't want high voltage traveling down through the center of the coil! It turned out a 1.5 inch PVC pipe cap slid into the cardboard tube nicely. I positioned this below the wire lead.

The 2 inch PVC cap slips on top of the secondary. The Tesla coil is done.

Step 6: Wire

High voltage wire is not to hard to find. You can find X-ray or laser cables online, or you can make your own HV cable by slipping lower rated wire into extra insulation like vinyl aquarium tubing. 

I like to salvage HV wire. Two good sources are old microwave ovens and old CRT monitors and TV sets. You will recognize the HV cables by their thick insulation (usually red). In CRTs and TVs, the HV cables connects the high voltage transformer to the picture tube. This cable ends in a black rubber suction cup-like connector. In microwave ovens, the HV lines connect the magnetron (source of microwave energy) to the microwave oven transformer. MOTs are heavy chunks of steel wrapped in wire, the heaviest component in the oven.


Microwave oven wire comes with quick detach spade connectors. In some cases these can be used as-is, but generally you'll need to clip these off and attach small alligator clips. Make sure to attach the clips snugly, and use electrical tape to seal the connections. You do not want high voltage arcing from loose connections.

For this Tesla coil setup you will need this amount of wiring:

from the NST to the spark gap: two wires about 12 inches long

from the spark gap to the lower end of the primary: 8-12 inches of wire

from the spark gap to the capacitor array: 6 inches of wire

from the capacitors to the upper end of the primary: 12 inches of wire

See photos for the hook-ups

One detail to consider: RF chokes can be had from Radio Shack and other retailers. These small chokes clip on over wires. I apply clip on chokes over the HV leads to the coil to prevent or dampen some of the radio frequency interference generated.

Step 7: Variacs and Vacuum Cleaners

"Variac" is a brand name variable transformer. Like xerox and kleenex, it's become the name of all variable transformers of the same type. A variac is a valuable addition to your Tesla coil system. The NST power cord plugs into the variac, while the variac plugs into a wall socket. Now you have an safe on-off switch for your coil, as well as the fuse protection of the variac. You can experiment with different voltage setting on the variac to find the optimal performance of your coil. (It may not be at 110 V AC, or the highest output of the variac). Variacs old and new can be found online. I have a cheap one made in China, but it works fine with my small coils.

Less useful but still helpful is an isolation transformer. These allow you to plug in some device and power it while isolating it from the wiring in your home or lab. I picked up an isolation transformer once at a Habitat for Humanity salvage sale for $15. It has four outlets and offers laboratory grade protection. 

If you build the pressurized spark gap, you'll want to take advantage of the greatly increased power output it offers. All you need to do this is a small vacuum cleaner. Even a handheld "dustbuster" will do, though a hose type floor vacuum comes ready-made with hose to connect to your spark gap. 

There's evidence that full blast air flow does not provide the best quenching effect to the park gap. One experiment you should try would be to vary the speed of the vacuum blower by means of a variac to determine at what pressure the spark gap performs best.

Step 8: Fluorescent Tubes

Nikola Tesla is the father of fluorescent lighting. In the 1890s he experiment with filament-less, gas-filled tubes and bulbs that glowed in the presence of high voltage. He saw these as a progression from the small, hot light bulbs invented by his rival Thomas Edison. Fluorescent lighting still works on principles established by Tesla: house current is stepped up by a small transformer to much higher voltages, which excites the mercury vapor sealed into the tube. The vapor fluoresces, or glows, using much less wattage than the incandescent system.

One of Tesla's longtime ambitions was to do away with complex and costly electrical wiring by providing power through an earth and air system. This is commonly referred to "wireless" or broadcast power, and his scheme has become one of the enduring legends of Tesla's unrealized dreams.  At the simplest level, Tesla's system would have consisted on high frequency electricity beamed into the air and similar waves of power pumped into the earth. With a simple aerial and ground wires, usable power could supposedly be tapped without the need for house wiring--or power company meters.

For various reasons, scientific and economic, it didn't happen and probably never will. But we can demonstrate Tesla's idea on a very limited scale. 

You'll need a few fluorescent tubes--any kind will do, even supposedly burned out ones. In this Instructable I used three 24 inch Cool White, 20W tubes. Two of these I bought new and one I salvaged from a broken fixture. 

To make these tubes stand up, we'll need support of some kind. It turns out our ubiquitous PVC plumbing has collars that work nicely for this purpose. Find 1.5 inch pipe connector, with one end a slip-fit, and the other threaded. The fluorescent tubes fit neatly in the threaded portion. The 24 inch bulbs will now stand.

Exposed to the output of our Tesla coil, the tubes glow mildly. This is because we've only completed half of Tesla's circuit. To get the best light, our bulbs have to be connected to the earth. 

In one PVC connector, drill a 6/32 hole near the slip-fit end. Attached a short 6/32 brass bolt and nut. Run a short piece of 18 gauge bell wire from the light tube's pin connectors to the 6/32 bolt. Tighten the nut to secure the wire. Now, on the outside portion of the bolt, clip on a length of ground wire. This can be any ordinary copper wire, run out of the house and hooked up to a ground rod. (I use a length of galvanized steel tubing driven into the ground outside my kitchen window. I run about 6 feet of 10 gauge house wire to the ground rod. The wire has alligator clips on both ends. One clip goes on the 6/32 bolt, the other grips the ground rod.)

Thus rigged, the tube glows much more brightly. This is a crude approximation of Telsa's idea. However, with the spark gap crackling, the Tesla coil top terminal spitting, and the vacuum cleaner roaring, this isn't a very practical way to illuminate the room. But it does illustrate the idea of wireless energy.

Step 9: Set-up and Connections

Here's a final description of the Tesla coil and fluorescent light set-up:

On a table near a window (or outdoors in clear weather), set up the neon sign transformer. Plug its power cord into your variac. Leave the variac unplugged from the house current until it's time to actually activate the coil.

Clip a length of high voltage wire to each output terminal of the NST.

Clip the other end of the wires to the opposite terminals (the copper pipes) of the spark gap. 

Connect the right hand terminal of the spark gap to the lower spade terminal of the Tesla primary coil. (All references to 'right hand' or 'left hand' refer to positions in the photos).

Connect the left hand terminal of the spark gap to one pole of the capacitor(s)

Clip a length of HV wire from the free end of the capacitor(s) to the upper spade terminal of the primary.

Position your fluorescent tubes in their stands about a foot away from the Tesla coil. Keep them clear of any wires, etc.

Clip a ground connection to the bottom free wire of the Tesla secondary. The end of the wire will need to be stripped of enamel (use sandpaper). USE AN EARTH GROUND ONLY! NEVER GROUND A TESLA COIL TO THE HOUSE'S WIRING, OR TO HOUSEHOLD METAL PLUMBING! You're asking for trouble if you do. Currents from the Tesla coil will travel through house wiring or pipes and damage sensitive electronics around your home--computers, phones, TVs, etc.  If a ground connection is not available, you can attach the secondary ground wire to the lower connection of the primary coil. This setup is often referred to as an Oudin Coil, or an auto-transformer.

Attach a separate ground wire to the fluorescent tube equipped with a ground connection.

Connect the input/output hose of your vacuum cleaner to the spark gap. 

Stand back by your variac. Power on!

Watch for shorts or sparking from your connections. Shut down at once it that occurs and reattach or tighten the offending connection. If the NST just hums and does not make any sparks in the spark gap, shut down at once to avoid burning out the NST. Make sure the spark gap is neither too wide nor touching. Try 1/8th of an inch or 1/16th inch gap to start with.

If everything is working, turn on the vacuum. Immediately the output of the coil will increase dramatically. Keep clear of the top terminal arcs. Check your light tubes for light. The grounded tube should be much brighter than the ungrounded one.

This set-up makes a lot of ozone and nitrogen oxide gas. This stuff is irritating and toxic so don't run the coil very long indoors. Outdoors is better on the lungs. Watch for signs of overheating, particularly at the spark gap during long runs. I never operate my coils for more than two or three minutes at most, and usually less than a minute.

You may notice I don't employ a safety gap or complicated chokes on this coil. Because the NST is current limited, it isn't absolutely necessary to use a safety gap, though it doesn't hurt. Because of the low output of the coil (and the use of clip-on chokes), I've never found it necessary to use chokes to prevent transients traveling back through the system and into my home wiring. Using a variac or isolation transformer protects the house. Drawing power through a GFI protected wall socket also adds a level of safety. If the system tries to draw too much, the GFI will trip.

I've been running these small Tesla coils in my house for 2 years and I've never had any serious side-effects. Be careful, and do what you need to do to protect your appliances and electronics.  

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