- This is a mechanical circuit,
which is to say it's a collection
of mechanical components that together
are a really robust analogy for electric circuits.
You've got resistors, transistors, capacitors,
ammeters, batteries, inductors.
It's really incredible what you can build with this thing,
and for me, it's really interesting to see my intuitions
about electrical circuits in physical form.
I can feel voltages and see currents.
It's called Spintronics. There's a link in the description.
They're not paying me at all to make this video.
I want to be clear about that,
but when I saw the Kickstarter,
I just knew I had to make a video
about it because I love an analogy.
Let's have a look at really simple circuit.
So this is the battery and this is a resistor
and the two are joined together by a chain.
When I charge up the battery by pulling this string,
the battery then discharges its energy into the resistor.
What's really cool is you can feel the resistance
like this one, which is a hundred ohms,
has a certain amount of resistance.
This one, which is 200 ohms, has twice that resistance.
You can feel it in your hands and look,
when you put the 200 in there, the chain moves
at half the speed of when the hundred was in there.
So the speed of the chain is like the amps of a circuit.
You can connect two resistors in series like this
and the chain moves even more slowly.
You can also connect resistors in series like this.
That's a bit counterintuitive at first,
because it kind of looks like they're in parallel
but this is what a series looks like
in a Spintronics circuit.
It's a bit like the analogy of water flowing through pipes
that you often hear when talking about electronic circuits,
but instead of water being pushed through pipes,
it's a chain that's being pulled
and it's pulling on different components.
This is probably a good time to address
the Veritasium-shaped elephant in the room.
You probably saw his video where he talks
about how electronic circuits really work.
It's the one about the circuit
that's a light second in length.
It's a really important video, actually two videos
'cause he did a followup that I recommend you watch
if you haven't already 'cause he clears a few things up.
They're important videos because it's important
to keep in mind what's really going on
beneath the models that we have
in our heads of how things work.
In the videos, he explains that the energy
in a circuit is carried by the electric
and magnetic fields that hug the wires
and components of the circuit,
and that kind of puts the lie to the hydrodynamic model,
doesn't it, because if you only have that
water flowing through pipes image in your head,
then you're thinking, okay, so I push water
at this end of the pipe, which pushes on the water
next to it, which pushes on the water next to it,
which pushes on the water next to it,
until this bit of water falls out the other end of the pipe,
and that bit of water can be used to do work.
And if that's the only model that you have,
then you might think, well, it's the same with electrons.
I'm pushing the electrons with a voltage
through this end of the wire, which pushes on the electrons
in front of it, which pushes on the electrons in front of it
until eventually these electrons at the end,
they can be pushed out of the wire and do work,
but that's not how it works.
The energy is carried by the electric and magnetic fields,
though to be fair to the electrons,
those electric and magnetic fields belong to the electrons,
so it is kind of fair to say that it's the electrons
that are carrying the energy.
It's just that they're not pushing on each other
in the way that the water in a pipe model
might lead you to believe.
But to make a broader philosophical point,
everything we know in science is just models.
Even quantum mechanics is just a model,
and it's wrong in the sense that
we know it's not a true description of the universe
because it doesn't play well with relativity,
but we still use quantum mechanics
because it's insanely accurate under most circumstances.
So you might have the water in a pipe model
of electric circuits in your head, but it's important
to know that underneath that, it's the electric
and magnetic fields that are carrying the energy.
But even that's a model, right?
I mean it's models all the way down.
But the point is when you're doing science,
you pick the model that is most appropriate
for the thing that you are doing.
This mechanical circuit model is good for intuition as well,
so let's explore some of those.
This is the Spintronics equivalent of a capacitor.
The more you try and turn it, the harder it gets to turn,
in the same way that the more you try and shove charge
into an actual capacitor, the harder it gets
and look, you can even see there's numbers on the dial
so you can see how charged up it is.
Actually this doubles as a volt meter,
so the numbers here are in volts.
A quick note on units, actually.
this says a hundred ohms, but actually
it's a hundred spin ohms, and this is spin volts.
What's really nice is they're all analogous
to the real things.
Like if we say that 10 meters of this chain
is equivalent to one coulomb of charge,
then all these other values like spin ohms
and spin volts and stuff written down here,
they're all equivalent then to the real thing.
Look what happens if I connect a resistor
and a capacitor to the battery.
You might think that these are in parallel, but look,
once the capacitor is charged up, the current stops flowing
through both the capacitor and the resistor.
That's because this is actually a series circuit.
So how do you achieve parallel circuits with Spintronics?
Well, you need junctions,
and actually junctions are more complicated
with Spintronics than they are in electric circuits.
Well, that's because they're incredibly simple
in electric circuits.
Look, here's a junction.
but this is a junction in Spintronics.
It's actually like a differential gear in a car.
If I hold this still, these two things move.
If I hold this one still, these two move.
That means whatever voltage you apply
to the bottom sprocket will actually come out
of the top two sprockets, and vice versa for any pair.
So remember when I had these two resistors in series,
they span at the same rate, just like an electric circuit.
The current through two resistors in series
will be the same, but look.
When I put them in parallel, the smaller resistor
spins much more quickly and that's as we would expect.
We expect a greater current
to flow through the lower resistor.
Remember, these capacitors are also volt meters,
so if I put a volt meter in parallel with this resistor,
then I'm measuring the voltage across the resistor.
That's pretty cool, isn't it?
One thing that's nice about a junction
is you can feel the voltage.
If I hold the top sprocket of the junction with my fingers,
I can feel it pushing against me.
That's the voltage.
Another cool thing about junctions
is that you can use them as a transformer.
In this circuit, this resistor is attached
to the junction twice, and this free spinning component
is attached to the remaining sprocket.
Powering this with my hand, I only feel half the resistance,
but the resistor turns at half the speed.
That's mechanical advantage.
So transforming voltages in Spintronics
is actually a lot easier than in regular circuits.
You can easily do it with DC,
whereas with electrical circuits,
you need alternating current.
There's even something analogous to an ammeter,
a device that indicates the amps in a circuit,
how much current is flowing.
It doesn't give you a reading.
Instead it gives you a pitch.
All along this rim, there are these serrations.
If I scrape it with my nail, you can kind of hear it.
Under here, there's a ridge that is connected
to this gramophone amplifier.
So the faster this thing turns, the higher the pitch.
(meter playing high-pitched tone)
If I add it to this circuit here,
that gives you a sense of what the current is like.
If I put it against this smaller resistor,
the pitch goes up.
In series with a capacitor, when I turn the switch on,
you can hear how current flows quickly into the capacitor
but slows down as it gets full.
With this setup, I can charge and discharge
the capacitor with these two switches.
This is probably my favorite component. It's an inductor.
This is what an electrical inductor looks like.
It's a coil of wire and how does one of these things behave?
For me, it was one of those things
that wasn't super easy to get an intuition for.
An inductor resists the buildup of current,
but once there is a current,
it resists the reduction of current.
It's all mediated by magnetic fields,
but with this Spintronics analogy,
it's all about inertia and momentum,
which is really intuitive.
Because of these weights around here,
it's hard to get the thing spinning,
and once it's spinning, it's hard to stop it.
Connecting an inductor to a capacitor
really gives a nice intuition about both of these components
and how they might behave in a circuit.
Because it's hard to stop current flowing
through an inductor, it can cause damage
to your circuit if it's not designed well.
In fact, the same thing happens with Spintronics circuits.
This is a switch here and when I turn it on,
the inductor receives power.
It eventually spins up and then watch what happens
when I turn the switch off.
It destroys the switch.
If you have a problem like that in a circuit,
then you add a resistor in parallel.
So now when I turn the switch off,
the inductor can dump its energy through the resistor.
By the way, if there's nothing connected to the battery,
it spins like crazy, which is exactly what happens
if you connect the two ends of a battery together.
The battery will overheat because of
the immense flow of current through it.
There's a little seatbelt-type brake inside
to prevent damage to the Spintronics battery
If there isn't anything connected,
or if you make a circuit like this
and you realize that there's nothing attached
to this part of the junction.
Actually that illustrates something quite quirky
about Spintronics, which is that components
are by default closed circuits.
If you charge up a regular capacitor and then remove it
from the circuit, it holds its charge.
But look, if I charge up this Spintronics capacitor
and then let go, it discharges.
So unless a Spintronics component is connected
to something else, it's probably connected to itself.
This is really cool. It's a transistor.
Look, no current can flow through this bottom sprocket
because of these clamps clamping down on that rubber ring.
But if I apply a small voltage to the top sprocket,
it opens up these clamps and the bottom sprocket can spin.
You might know that you can use
a transistor as an amplifier.
So a small voltage applied up here
can lead to a large current flowing down here.
Here it is in a circuit.
This capacitor that's acting as a volt meter here
is showing the small change in voltage
that I'm supplying with my fingers.
This capacitor/volt meter is showing
the much bigger amplified voltage across the load.
You can even make a diode by connecting
a transistor to itself via a junction.
Look, it will spin one way but not the other,
just like a diode.
So let's see if we can build some interesting circuits.
This is a peak voltage detector.
So I can apply a voltage manually with my hand
and this capacitor/volt meter is showing me that voltage,
but this capacitor/volt meter is showing me
the peak voltage.
Because this pair of components as a diode,
the capacitor won't discharge when I let go over here.
What's brilliant about all this
is I can feel it as it happens.
It's really intuitive.
Spintronics actually comes with a dedicated diode component,
so look, here's a simplified version of the circuit.
The diode works by a simple ratchet mechanism.
You can turn it in one direction but not in the other.
This is a high pass filter, so I can send a voltage signal
into the circuit with my hand by spinning this thing.
I can do it at high frequency,
I can do it at low frequency,
and a high pass filter will filter out
the low frequency signals,
leaving the high frequency signals.
But how does it do that?
Well, this is an inductor and it's in parallel.
You can think of an inductor as a short circuit
for low frequencies and DC.
After all, DC is just very low frequency.
So if I try and turn this thing with a constant force,
once the inductor spins up, then it's basically
a short circuit and none of that DC signal
will get through to the ammeter.
An inductor is a bit like a circuit break
for high frequency signals.
So if I move this thing back and forth quickly,
then we do hear it at the ammeter
because it doesn't have much of an effect on the inductor.
To make a low pass filter,
you replace the inductor with a capacitor.
A capacitor is like a short circuit for high frequencies
but a circuit break for low frequencies.
This is an oscillating circuit if you ever need AC
for something in your Spintronics world.
Here's a fun story.
When I was 18, I wrote a scientific paper
and got it published in a journal.
That sounds more impressive than it actually is.
At the time, my physics teacher, Mr. Parkinson,
said something like, "You know that idea you had?
I could probably get it published
in Physics Education Journal."
They had this section for student submissions,
so that's what it was, but I still like to tell people
that I got a paper published when I was 18.
But the idea was this.
If you get a charged capacitor and you connect it
directly to an equivalent uncharged capacitor,
then half the charge will leave the charged capacitor
and end up in the uncharged capacitor.
They reach equilibrium with each other.
What's surprising is if you calculate the energy
of those two capacitors after you connect them,
you have exactly half the energy you had before.
But where does that energy go?
I had this idea that instead of thinking about capacitors,
think about tanks of water.
So you have one tank of water that's full
and another that's empty, and it's connected
at the bottom with a pipe.
When you open that pipe, water will flow
from the full tank to the empty tank until they're equal.
But actually because of momentum, there will be an overshoot
and the second tank will go slightly above halfway
and then it will relax down again
and it will oscillate backwards and forwards
until they have an equal amount of water in both.
But every time the water sloshes backwards and forwards,
it loses energy through friction
and that's where the energy goes.
Electrons in a circuit don't have momentum
in a meaningful way, but the wires between the capacitors
do have stray inductance, so I wanted to see
if the same thing would happen with Spintronics.
If I charge up one capacitor and connect it to another,
will they oscillate backwards and forwards
until they both have half the charge, and will it work
with just the stray inductance in Spintronics?
And look, there they go.
They do oscillate backwards and forwards.
Not for long because there's a decent amount
of stray resistance in Spintronics as well,
but it's nice to have my paper from 1997 validated.
What about a full bridge rectifier?
Can we make one of those?
A full bridge rectifier converts AC to DC with four diodes.
As you can see here, as I supply AC here,
you get DC at the load here.
It's not very smooth but you can fix that
with a couple of capacitors.
I only have two diodes, which is why I'm showing you this
in simulation, but luckily with Spintronics,
you can make a full bridge rectifier with just two diodes.
The reason you can do it with Spintronics
but not with regular electric circuits
is because with Spintronics, it's really easy
to change the direction of current,
which it isn't in normal circuits.
Depending on how you chain up a junction,
you can flip the direction of current flow.
What about computation?
Could we use this to build a computer of some kind?
Well, look, here's a flip flop.
When I turn this switch on,
it registers at this volt meter
but when I switch it off, the volt meter stays on.
This switch puts the volt meter back to zero,
and so I can flip flop between the two.
This is an XOR logic gate.
If switch one is on, we get a reading at the ammeter.
If switch two is on, we get a reading at the ammeter.
But if both switches are on, we don't.
So maybe we could use this stuff to build
a binary adder like I did with my water computer video.
If I ever get that working, I'll make a follow up,
but in the meantime, is there anything I've missed?
Is there a really cool circuit
that you would like to see built with Spintronics?
Let me know in the comments.
Thanks to Paul from Spintronics
for all his help with this video.
And thanks to Mr. Parkinson for being
just a brilliant physics teacher.
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