by Bob Bridges
There are a few good reasons to use two coils instead of a single coil, such as;
- Sharing a session with someone else who is working on the same issue. Makes for “double duty” of the machine and is much less boring when you have company.
- Hitting two different targeted areas without having to move the coil. This is especially useful if you are doing your PEMF session while sleeping, but even while awake, like when you are watching a movie, it means you don’t have to pay attention to when your session changes from one issue to another or interrupt what you’re watching.
- For working on metabolic issues, like improved blood circulation, two coils spread the magnetic field over more of your body.
- Two coils opposing each other (e.g.: one beside your abdomen on the left and on the right, or one on either side of your knee) produce a stronger magnetic field and puts that area of your body in the strongest part of the magnetic field.
That last one…Really? Well, let’s examine that. It will involve some math, but I promise, it will be easy math.
First, some basics.
In nature, everything has a “resistance” to the flow of electricity (current) through it. Some things are so resistant to the flow of electricity that it cannot get through unless it’s in very large amounts, like glass or rubber. These are called insulators. At the other end of the spectrum, you may have heard of work being done to create a “super conductor”. This is creating a structure that approaches zero resistance…and is extremely hard to do.
The “resistance” to the flow of electricity is measured in Ohms (named for German physicist and mathematician Georg Simon Ohm [March 16, 1789 – July 6, 1854 C.E.] who conducted research in electricity in 1826 and 1827, publishing the results that came to be known as Ohm’s Law in 1827).
The flow of electricity itself is referred to as “current” and is measured in Amperes (named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electromagnetism).
Now we need one more thing to have an electrical system. Something to force the electricity to flow. It’s called the “potential difference”, or voltage, and is measured in volts (named after the Italian physicist Alessandro Volta [1745–1827]).
Ohm’s Law describes the relationship between these three key physical quantities: voltage, current, and resistance. It represents that the current is proportional to the voltage across two points, with the constant of proportionality being the resistance.
This is expressed with the equation:
R = V / I
R represents the resistance of the conductor in ohms,
V represents the voltage measured across the conductor in volts, and
I represents the electrical current in units of amperes.
The voltage output from the amplifier at a given frequency and volume setting is constant. Each coil has a set amount of resistance based on the properties of the wire used to make the coil. The resistance of each coil is constant, but we can double that by adding the second coil to the system.
We can write Ohm’s Law in another way so that we can better visualize what happens to the current if we double the resistance when the voltage is constant:
V = IR
According to the formula above, if the resistance (R) doubles by adding a second coil to the system, then the current (I) would have to be ½ compared to the system using a single coil in order for the voltage to remain constant. It’s not that the current is splitting with half going to each coil, it’s that the overall system current is half because of the doubled resistance of the two coils.
That was pretty easy math, right? It will get a little harder, but not much.
Here is the formula for the magnetic field strength of a coil:
B is the magnetic field strength (measured in tesla, which can be converted to gauss by dividing by 10,000).
N is the number of turns in the coil (how many times the wire circles the coil).
µ sub o is the permeability in free space of the coil’s core (this is a constant that has been well established which varies depending on the core material, in our case, air).
I is the current going through the coil in amperes.
R is the radius of the coil. (Don’t confuse this with the R for resistance in Ohm’s Law above.)
You could calculate the formula if you want, but you don’t even need to do that to see there is a relationship between N and I with B (we are not concerned with µ sub o because it is a constant). If the value of N goes up, so does the resulting value of B. Likewise, if the value of I goes up, B goes up. You can test this out by filling in “easy math” numbers and calculate the resulting B, then increase the N value and recalculate to see what happens to B. Likewise, any of the other values can be increased or decreased (except for µ sub o because it’s a constant!) to see what happens to B and see the relationship of the various values. To create a high magnetic field strength, you must have high current (I) and/or a large number of turns (N). If your current is low, you must have an even greater number of turns to make up for it, and vice versa.
Now, let’s go back and review our first equation with some real numbers.
Remember, V = IR.
The resistance of a single coil is .8 ohms, and assuming we are driving the system at the highest currency the coil’s wire will carry, we use 20 amperes for the current. This results in —
V = 20 * .8
V = 16 volts
From here forward, we can now use the constant of 16 volts to see what happens when we add the second coil. Our two coil formula would be –
16 = I * (.8 + .8)
I = 10 amperes
Look at that, one half of the system current a single coil yields!
Now we put that understanding in the magnetic field strength formula and find that N, the number of turns, doubles by adding the second coil but I, the current, is cut in half resulting in no change to B.
Ah, but wait…
There’s something I haven’t told you, and it will make all the difference.
.8 ohms is too low of a resistance for the amplifier, which needs a minimum of 1 ohm. To rectify that, we added .5 ohms of resistors inside the chassis giving the single coil system a total resistance of 1.3 ohms, comfortably above the amplifier’s minimum requirement.
Now, if we go back and add that factor to Ohm’s Law for a single coil system we get –
16 = I * (.8 + .5)
I = 12.307 amperes
Let’s compare that to the two coil system –
16 = I * (.8 + .8 + .5)
I = 7.619 amperes
Notice the current of the two coil system is well over half that of the one coil system.
Now we can revisit our formula for magnetic field strength
and see that when we add the second coil we double the number of turns N but we don’t cut the current I in half, resulting in B being a higher value, making it a stronger magnetic field strength. Running with two coils in close proximity to each other actually produces a stronger magnetic field than running just one coil!
This is the first benefit for using two coils.
Now, for the second benefit…
In an electromagnetic coil, the very strongest magnetic field strength is right in the center of the coil, both in the center of the circle the coil forms and in the thickness of the coil. It’s what we call the “sweet spot” and is why we make our coils as large as we do…so you can put body parts through it so that body part is in the sweet spot, or area of highest magnetic field strength.
Now imagine two coils stacked together, magnetically acting as a single thicker coil. The sweet spot is in the center of the circle and the center of the stack, right where the two coils meet. As the coils are separated, the sweet spot starts to elongate while remaining between the two coils. This sweet spot will continue to stretch and weaken between the coils while two spots, at the center of each coil start to regain their strength until at some point of separation the spot between the coils is reduced to ambient strength while the two growing sweet spots regain their full strength and they once again act like two independent coils. How far must they separate to no longer have a sweet spot between them? Not very far, but farther than a body is thick. That means when you have one coil on each side of you oriented flat to your body, the sweet spot is in the core of your body rather than off to one side like it is when a single coil is used.