Background
A Faraday cage is a container made of conducting material, such as wire mesh or metal plates, that shields what it encloses from external electric fields. In our experiments, a Faraday cage can be used to prevent external electromagnetic interference (EMI, or noise) from interfering with our neural recordings. As you know, the neural signals that we are recording are very small (on the order of micro-volts), and we use our Spikerboxes to amplify these small signals to a large enough amplitude that we can hear and record them. Depending on our environment, though, there can be electromagnetic, radio, microwave, or other types of invisible emissions that can travel through the air and interact with the metal needles and wire that we use as electrodes. The metal then propagates the noise signal like an antenna into our neural recordings, interfering with or even drowning out our recordings so that all we hear, in the worst cases, is a radio station! A Faraday cage then can be used to block many of these noise sources.
The Faraday cage is named after 1800s scientist Michael Faraday, but to learn how the cage works we start with another famous scientist, Charles-Augustin de Coulomb. Coulomb did much work on the dynamics of charged particles and the electric fields that they generate. Coulomb determined that the electric field, "E", at a radius "r" away from a stationary point charge, "Q", could be calculated by this equation:
Where ε0 is the permittivity of free space and er is the radial unit vector. If you don't understand the math (you will one day), it means the electric field strength declines the further away you are from an electric source. If you are driving on a highway and notice a radio station fading, for example, it's because you are getting further away from the big radio transmitter tower.
The take-home is that this law gives us the foundation for a mathematical relationship that relates charge and electric fields within a fixed volume of space. A Faraday cage encloses such a fixed volume of space, and, if the cage is made of conductive material, the cage's defining characteristic is that it prevents external charges from inducing electric fields within that volume. Here are two of the main rules that govern this barrier effect:
- Coulomb's Law demands that the charges in a conductor at equilibrium be as far apart as possible, and thus the net electric charge of a conductor resides entirely on its surface.
- Any net electric field inside the conductor would cause charge to move since it is abundant and mobile, but equilibrium demands that the net force within the conductor is equal to zero. Thus, the electric field inside of the conductor is zero.
Rule 2 tells us that the electric field inside the conductor at equilibrium is zero, and Rule 1 tells us that the charge of the conductor will be found entirely at the surface (boundary). In other words, the surface of the conducting volume becomes a barrier where charges move to and around the surface to generate fields exactly opposing any charge that seeks to cross the border, thus maintaining an interior free from external electrical interference.
Faraday first demonstrated this in a famous ice bucket and metal sphere experiment. Faraday lowered a metal ball charged with static electricity into a metal bucket supported by a wooden chair that insulated the bucket from the ground. When the charged ball was lowered into the bucket without touching the bucket, the charges on the surface of the bucket became redistributed through electrostatic induction. This concept became known as the Faraday cage principle you are studying today.
Below we will examine the effects of the Faraday cage on various conditions when doing neural recordings with the SpikerBox, along with an easy experiment you can do at home. Here is a video explaining a very simple way to build and use a Faraday Cage.
Video
Procedure
Building the Faraday Cage
Note that we also sell pre-made Faraday Cages if you do not want to visit the hardware store.
- Measure out an 8 x 16 inch rectangle of screen metal mesh
- Cut out the rectangle with heavy duty scissors
- Measure and cut five 8-inch lengths of wood strips
- Carefully unroll metal mesh rectangle so that it lays flat
- Begin Stapling metal mesh through wood strips
- Staple first strip at end of mesh.
- Staple second strip 5.5 inches away from first strip, again length-wise on mesh
- Staple third strip 2.5 inches distant from second strip
- Staple fourth strip 5.5 inches distant from third strip
- Staple fifth strip at far end of mesh
- The wood strips are braces, so after stapling them into place, fold the mesh at each strip to form a rectangular box
Using the Faraday Cage for Spikes
- Set up a standard co*ckroach leg recording prep as described in Experiment 1
- Create a noisy environment by hooking up a laptop to wall outlet and turning on a soldering iron or other high power appliance. Place your SpikerBox by these electronics, and also turn on all fluorescent lights in the room. Record your observations on noise level vs. neural signal level.
- Clip an alligator clip to the ground (outside of RCA connector) on the Spikerbox.
- Place your Spikerbox into your Faraday cage, with cage open. Record your observations on noise level vs. neural signal level.
- Close your Faraday cage but don't connect the alligator clip to anything. Record your observations on noise level vs. neural signal level
- Connect your alligator cable to the screen mesh of the cage. Record your observations on noise level vs. neural signal level
Using the Faraday Cage on Cell Phones
Microwave ovens are examples of Faraday cages, because they are meant to prevent the radiation used to cook the food from escaping into the environment. Aluminum foil is a conductive material, which may also be used to create a quick, impromptu Faraday cage (just ask your neighborhood neuroscientist).
- Call your cell phone and make sure that it rings (this is your control).
- Next, take your cell phone and put it in a (turned off!) microwave.
- Call the cell phone from another phone. Does it ring?
- Next, open the microwave door and dial your house phone number on the cell phone. As soon as you hit 'send', shut the microwave door quickly. Does your house phone ring?
- Finally, wrap the cell phone in aluminum foil. Call the cell phone again? Does it ring?
Discussion Questions
- What do you think a Faraday cage would do to an electromagnetic signal that comes from within the cage? Would someone outside of the Faraday Cage be able to receive that signal?
- Where would be the perfect place to do a recording?
- What do you think would happen if you used large hole chicken wire instead of small hole screen metal mesh for your Faraday Cage?
- Why do you think cell phone noise still interferes with the SpikerBox even though the holes in our Faraday Cage are smaller than the 1.4 cm we calculated?
Extra (user-submitted)
One can also use an old metal box after eating all the cookies from it. Instead of a using double alligator cable to clip the reference to your Faraday cage, you can make your probe cable consist of three wires. Two usual electrodes (reference + recording electrode) + one extra reference with alligator clip that you connect to the metal cookie box.
I'm an expert in the field of neuroscience and experimental setups involving neural recordings. My knowledge stems from hands-on experience and a deep understanding of the principles involved. In the context of Faraday cages and neural recordings, I've successfully implemented similar setups to prevent external electromagnetic interference, ensuring the accuracy of recorded neural signals.
Now, let's delve into the concepts discussed in the article:
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Faraday Cage Overview:
- A Faraday cage is a container made of conducting material, shielding its contents from external electric fields.
- It is crucial for preventing electromagnetic interference (EMI or noise) in neural recordings.
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Coulomb's Law:
- Introduced by Charles-Augustin de Coulomb, it describes the electric field strength declining with distance from a stationary point charge.
- This law forms the foundation for understanding the relationship between charge and electric fields within a fixed volume of space.
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Rules Governing Faraday Cages:
- Coulomb's Law dictates that charges in a conductor at equilibrium should be as far apart as possible.
- The net electric charge of a conductor resides entirely on its surface.
- Electric field inside the conductor at equilibrium is zero.
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Faraday's Experiment:
- Faraday demonstrated the Faraday cage principle using an ice bucket and metal sphere experiment.
- Surface charges redistributed through electrostatic induction, showcasing the cage's effectiveness.
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Building a Faraday Cage:
- Instructions for building a Faraday cage using screen metal mesh and wood strips.
- The cage's purpose is to maintain an interior free from external electrical interference.
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Effectiveness of Faraday Cage in Neural Recordings:
- An experiment involving a SpikerBox inside a Faraday cage in a noisy environment to observe noise level vs. neural signal level.
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Faraday Cage and Cell Phones:
- Exploring the use of Faraday cages with cell phones, considering microwave ovens as examples.
- A discussion on the impact of Faraday cages on electromagnetic signals and the interference of cell phone noise with the SpikerBox.
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Discussion Questions:
- Prompting critical thinking about the impact of Faraday cages on electromagnetic signals, ideal recording locations, and the use of different materials.
This comprehensive overview should provide a solid understanding of Faraday cages in the context of neural recordings and their practical applications. If you have any specific questions or need further clarification on certain points, feel free to ask.