Oldest Radio Transmitter: Spark Gap

The simplest and oldest radio transmitter: The Spark Gap Transmitter

How simple can a radio be? How about a coin and a 9 volt battery?

Any radio transmitter – FM or AM radio station, walkie-talkies, garage door openers, cell phone, remote control toys – operates on a frequency that our government approves for different things.

When you’re listening to an FM 98.7, that station is transmitting at a frequency of 98.7 MHz. The government assigned that frequency to the station, and for the area covered by that station and approved by the government, nobody else is allowed to transmit on it. The frequency might be allocated to another radio station in another city that’s far enough away so that they don’t interfere with each other. And the government limits the power the radio station can use to transmit so it doesn’t go too far.

When everyone follows the rules, it works great, and we can all listen to the radio, talk on our cellular phones and even use powerful amateur radios without messing each other up.

But it wasn’t always that way.

In the earliest days of radio, they had not figured out how to nail things down to one frequency. They used a kind of radio that just made big huge sparks, and they noticed that it could be heard far away. You could not transmit sounds like voice and music, but the spark noises could be used for morse code. Problem is, those sparks are messy, and they transmit across lots of frequencies, not just one. It worked, but you couldn’t have a lot of people transmitting. They would all be talking over each other. That kind of transmitter is illegal today. This is the kind of radio that was on the Titanic, and there’s a great video and explanation of how it works here.

Spark Gap Transmitter from Hammond Museum of Radio

This a picture of one of those radios at the Hammond Museum of Radio in Guelph, Ontario.

As long as we keep it really low power, we can make a very simple one that won’t get us in trouble with the law. We learned it from this web site.

 Equipment required:
  • A 9 volt battery
  • A coin – quarters work best because of the ridges on the circumference
  • An AM radio to hear the signal.
Here are the steps:
  1. Turn on the AM radio, and go to a station where there’s no signal – just hiss.
  2. Hold the coin on one terminal of the 9V battery, and slide it so that it just flicks on the other terminal.

You hear a click or spark noise from the radio? That’s because you generated a spark across the battery terminal when the coin touched. You might even have seen a spark. If you tapped back and forth the right way, you could even generate morse code.

To prove how sloppy it is and that it’s not just on one frequency, tune the radio to another AM station where you just hear hiss. Flick the coin across the terminals, and you’ll hear it again. It’s crossing a lot of frequencies.

Good thing we don’t have a lot of power, or we’d be messing up the whole neighborhood’s radio listening.

Short circuit: explain why this experiment is a bad thing

Yes, we can actually do a circuit diagram for rubbing a quarter across the terminals of a 9V battery.

Short circuit when you close the switch. This is a bad thing.

In addition to creating very messy radio signals, connecting the terminals of a battery – or any other power source – directly together is a Really Bad Thing, and it’s called a short circuit. There should always be something between the two that uses the electricity – and that’s called a load. Most electrical diagrams will at least show some kind of resistor.

What happens in a short circuit?

  • The battery or power supply will deliver as much power as it possibly can.
  • This can cause the battery to overheat, and in some cases, enough to cause a fire
  • The amount of electricity (the electrical current) flowing through the wire can be high enough that even the small amount of resistance in a small wire will cause it to heat up – sometimes enough to cause a fire.
  • And worst of all, 9V batteries are not cheap, you’re going to wear it out really fast.

Advanced: demonstrating the short circuit for older Scouts.


For older Scouts – we recommend at least Scouts and maybe even Venturers – you can demonstrate this by substituting a 9V battery cap for the coin.

9V battery cap. Strip the ends of the wires and make a short circuit – and be careful! They get hot fast!

The coin is like a nice thick piece of wire. It has hardly any resistance, so we don’t feel any effect of the current when we’re holding it and touching the terminals of the battery

Tap together the wires from the 9V battery cap with a new battery, and you’ll get the same effect on the AM radio. Keep on doing it, and you’ll notice those little wires heating up… and not just a little.

Another name for it: interference

If you’re in the car and the radio station you’re listening too gets all scratchy when you go near power lines – well, it’s pretty much the same effect. Sometimes an old electric motor, power supply or something will also cause noise on your radio or TV.

The effect of the interfering signal should never goes far. You drive past the power lines, and it stops. If it reached a long way, it would violate communication regulations.

Government regulations limit how much of this interference any device can generate. Pretty much any electrical device you buy has to be tested and certified for maximum electromagnetic  emissions (fancy word for radio and electrical signals).

Is this how Morse code works today?

No! Morse code today uses a radio wave on a single frequency. That’s why ham radio operators refer to Morse code transmission as CW – Continuous Wave. You can learn how that works in this project on AM radio transmission.

Adding some math and science

We covered the short circuit issue.

Show them a simple electrical circuit diagram – battery, switch. Short circuit. Remind them that it’s bad.

What does 640 on the AM dial mean? Or 98.7 on the FM dial?

This is a great opportunity to introduce  radio waves, the speed of light, and the very subtle difference between sound and radio waves – and how to we move from one to the other.