- 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. 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. VPNs can be really useful. I recommend the one I use,
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