In electrical circuits made with normal conductors and insulators, e.g. copper wires and organic polymer insulators, it is possible to ignore the currents that branch from the wires and close through the insulators, because they are extremely small (e.g. in the femtoampere range), but frequently it is not possible to ignore the voltage drops along the copper wires, which are caused by the resistances of the wires, and which may be in the millivolt range.
As much as I love academia, I really wish there were more instructors with real experience that could say things like, "for math purposes assume a zero-loss conductor" while still emphasizing that real life is not so simple.
We did have a healthy incubation program, where instructors helped companies with product development, so perhaps that is what brought the real-world experience.
Generally, the biggest problem with math, physics and chemistry is that if you miss just one lesson (no matter if you physically miss it, or in worst case miss out understanding a concept for a couple minutes), you can easily end up "left behind" for years. With history, arts, languages, a number of computer science topics or most of biology you can either skip out on a certain subject or catch up easily on your own, but not getting a specific part in math/physics/chemistry and lacking the resources to follow up is setting oneself up for exponential learning gap later on.
The societal problem is that we've not set up schools or universities to deal with this "failure mode". Privileged kids can have after-school teaching or siblings helping out, but most will sooner or later resign and hate these subjects for the rest of their lives - and that is incredibly sad.
Why do I get a sneaking suspicion that few in the machinery of education want this, as it is a fast-track to continuous accountability?
A nice-to-have addition to your table saw is a big red stop button you can reach with your foot so you don't have to take your hands off of your work pieces to shut the saw down if things have gotten more interesting than you expected them to. Three wire control makes this super easy to add (once you've found the big red mushroom-head n/c switch, of course).
[0] https://twcontrols.com/lessons/control-wiring-3-wire-control...
White means "the red glass lens in front of the incandescent bulb shattered again".
Related, from train signals:
> The invention of the track circuit in 1872 by William Robinson permitted automatic block signals. Most simply, a low voltage battery current runs up one rail and down the other. As long as current flows from the battery through one rail to the relay and back through the other rail, the relay remains energized and routes current from another battery to the green lamp of a clear signal. But when a train is present, the circuit is short circuited by the steel wheels and axles of the cars; a broken rail or an open switch will also break the circuit. As a result the relay is opened and the signal displays red. The system is arranged so the failure of any component results in a red indication or a dark signal (which must be interpreted as the most restrictive aspect that signal can display).
https://www.trains.com/trn/train-basics/abcs-of-railroading/...
I remember a house at the bottom of a steep mountain grade that got plowed into on a regular basis especially during that time.
EDIT: hmm... seems freezing might still be a way to cause failure?
In other words, the brakes would apply on loss of pressure, instead of via pressure.
Do you have any details on that? Or a source I could read? I'm pretty curious now.
I drove trucks in the military - specifically, FMTV, HEMTT, HMMWV, and 5 tons. None of them had the reverse brakes you describe.
At the time it was an acceptable trade-off; later military vehicles have more in common with trucks than tanks, I suspect.
The brakes, unboosted, are supposed to still be capable of stopping the vehicle. This is also why hybrid and electric vehicles with regenerative braking, that normally use brake-by-wire with a pedal feel simulator, fail through to using the PFS as a direct unboosted hydraulic cylinder.
Losing all brakes in a hydraulic circuit failure is just because those trucks are so old they only have single circuit hydraulic brakes. Part of the A3 upgrades was changing over to a modern dual circuit hydraulic system.
So basically, the issue is that medium trucks are in a weird in-between. Not heavy enough to require full air brakes, but heavy enough that some of the safety assumptions we make about light vehicles are potentially less true.
For example, cars have "fail open" brakes but have independent cylinders so that it is relatively hard to have all four wheels fail at the same time (older cars had single master cylinders) - and one of the tradeoffs is that people want cars to get going immediately and not wait for brake cylinders to "charge up".
As more and more things get computerized, you have more options for "intelligent failover" - anyone who used to drive an old manual car knew you could slow down using the engine - but now the computer could detect brake failure and do things for you to help recover.
And as seen from other comments, the modern US military prioritizes "on-road safety" over "under ongoing attack ability" - partially because of changing strategies I suspect, but also from the realization that the US military kills more members of the US military each year via accidents, etc, than anything else.
You can use engine braking in automatic vehicles too, by simply changing to a lower gear.
An automatic one will not, and will hold the current gear until car speed falls within shifting range.
Ask me how I know :)
This of course meant the entire cars safety and stability control systems, ABS and traction control were all disabled. It drove like a tank.
The fix, obviously, was to replace the brake booster primer pump (hybrid car). It had worn enough to no longer recalibrate. Still perfectly functional, and would have kept working for a long time, bit it wouldn't quite meet the factory expected parameters so it failed it's test.
It appears that ZF 4HP20/4HP22 gearboxes can do this an unlimited number of times without apparently being damaged, but I'm not trying it on either of mine.
And emergency brake that's wholly separate circuit.
If you asked for “signal lights” I could see it. But not traffic.
In the end the known unknowns aren't that bad, just assume laziness and choose whatever checks the least boxes. But the unknown unknowns are regular culture shocks. Like pedestrian lights not being considered traffic lights.
So many not what most people would pick, but what Americans would pick?
It used to be that you had to know english in order to understand them as well...many of the signs used to use the words "walk" and "don't walk"
In many cases the US were the first to mass-manufacture things to some kind of standard, so the question is really: why did the rest of the world choose a different standard. Often they have a good reason (better design, maybe), but the US also has a good reason for not switching (inertia, existing tooling, people understand the existing standard).
The US was the first place that traffic light and crossing signals were installed. The reason they didn't go with the standard the rest of the world uses is that it didn't exist.
In fact, a theoretically better standard could actually be worse in some situations if people are accustomed to a different standard. Imagine if America had settled on red for go and green for stop, and then tried to change the standard to match the rest of the world. It would be a calamity as some drivers continued to adhere to the old standard that they knew.
Stop signs were originally yellow, so such a change isn't unprecedented, but in that case they also contained the word STOP so the colour wasn't critical to understanding the sign. Standards can more easily change when there is backwards compatibility available. More recently, some jurisdictions have started adopting traffic light shapes (square = stop, diamond = caution, circle = go) but retain coloured lights for backwards compatibility.
Like, the WALK / DONT WALK crossing signals that were around in my youth were slowly replaced with the HAND / WALKING PERSON ones, I'm sure because they're better for people who don't read English.
But also there is a cost to change, and it's often not worth doing if the benefit from the new system or standard isn't a lot better than the old.
Why doesn't the rest of the world (outside the US) have a 3rd middle brake light (CHMSL), as was mandated in the US in 1986?
My guess is because, despite measurements of ~22% crash avoidance at the time, that number turned out to be between 0% and 4% in practice. Maybe the rest of the world didn't care about 4%, or maybe they thought it was as stupid then as it turned out to be.
But the CHMSL standard in the US has refused to die.
This resulted in premature conclusion errors. They went to the ATM to get money. They got their money so they left... forgetting their card.
When ATMs were updated they fixed the design error. Now the card popped out and had to be removed before money would dispense. This resulted in a spike of people leaving with money hanging out of the ATM because they had been trained that removing their card was the end of the task. (Which is why money now gets sucked back into the machine if it hasn't been removed fast enough).
The point of the anecdote (other than that you hire HCI experts before implementing an interface) is that implementing a new and objectively better system doesn't necessarily result in an objectively better outcome when replacing an incumbent worse system.
Case in point: Ma Bell. And things like signing the credit card receipt.
Traffic lights? Not so many actually?
Seems like you understand it fairly well. I can't speak for Canada, but the U.S. mostly ignores how other parts of the world do things.
What you're thinking of is called procedural memory, which helps you perform a task without being consciously aware of it (hitting a fast baseball, looking left/right when crossing a street).
Dr. Pavlov's seminal book is generally translated as Conditioned Reflexes. Saliva in response to a bell is a reflex, and it's an acquired one.
Not falling flat on our face when we step forward is also reflexive, as is catching something thrown to us. Both are acquired through rather lengthy processes.
There are many similar examples.
Back in 1649, when Descartes formulated the concept of a reflex, it was used to describe lower animals, to support his notion that they were automata without a mind on their own. The word reflex originated from the "reflection" of the sensory input into a response. The physiological backgrounds were not yet known.
In the early 19th century, Hall narrowed the definition of a reflex to be a "involuntary action of a muscle or gland in response to the stimulation of a receptor neurone which does not depend on the existence of consciousness".
Sherrington, in 1904, narrowed the definition further, introducing the concept of the "reflex arc", a hardwired pathway between receptors and effector muscles.
Pavlov later did a strange thing and widened the definition, which is the source of some confusion. Nowadays, Pavlov's Conditioned Reflexes are mostly called Conditioned Responses, to avoid this exact confusion. NB: there are people arguing that conditioned is a mistranslation, and that it should mean conditional, which makes more sense in the context.
Pretty much every current publication uses the term "reflex" for involuntary, inborn responses, detected by receptors, transported to the spinal cord by afferent nerve fibers, processed there, and the motor signal sent through the efferent nerve fibers to the effector muscles. I only found the term "conditioned reflex" in papers discussing Pavlov's experiment.
However green on walk would be nice.
Very visually distinct, walking and standing, and green/red.
My working theory had been green figure means cross, red figure means cross with caution, like a flashing red hand in the US.
I agree with you. When I'm in the US I keep scanning for a green light, so it takes me a while to find "the white man" telling me it's OK to cross.
Just figured I'd mention that.
Only if you define the rest of the world to mean the very small percentage of places in the world you have been.
From Wikipedia:
In some countries, instead of "don't walk", a depiction of a red man or hand indicating when not to cross, the drawing of the person crossing appears with an "X" drawn over it.
Some countries around the Baltic Sea in Scandinavia duplicate the red light. Instead of one red light, there are two which both illuminate at the same time.
Maybe Australia? You don't remember if you've been to Australia?
Congratulations! I've never been so drunk that I didn't know which continent I was on.
> The icons displayed (hands) are confusing too: the rest of the world uses an icon of a man walking or standing.
Is a hand held up, palm-out, not a signal for "stop" in places other than the US or Canada? I wouldn't assume it's universal, but I'd expect it's got broader reach than that.
Besides, again, I'd personally assume it'd be better to have very different icons for "walk" and "don't walk". Not two different depictions of a person.
How far down the line would the supply voltage run (or how far away would a train complete the circuit) before resistance losses degrade the system?
The maximum length of a block with this system (versions of which are still used today) varies greatly depending on local conditions. I've seen 2k to 10k feet as a general performance range.
The transmitter and receiver are wired in parallel and connected to a power supply through a series resistor. When the receiver receives the signal from the transmitter, it pulls the supply voltage down, which then causes the transmitter to stop transmitting, which then causes the receiver to stop pulling the supply down, and the cycle continues. The end result is that if there's no obstruction, the supply voltage pulses and the system checks for this. If there's any other type of condition, the signal is either 0 or VCC. Basically, the system is designed so that false positives are extremely unlikely since that's the worst case scenario.
There's some capacitors in the transmitter/receiver to control the frequency.
edit: it gets you protection against one of the two failing closed, duh
Strictly speaking, there is no Break key in the same sense that there is no SysRq key—there are only special Break and SysRq scancodes that AT and PS/2 keyboards send instead of Pause and Print Screen when Ctrl or Alt respectively is depressed (thus the combined labels), while USB keyboards don’t even have those, they just send normal modifier sequences. But I don’t think that’s what you had in mind :)
I would be willing to bet that none of the ten most popular/best-selling computers in the country for the last 5-10 years have had a key that says “break”.
I'll take that bet and offer up a break key from my active keyboard if I'm wrong.
For example, on most modern Thinkpads, Pause is Fn+P, and thus Break is Ctrl+Fn+P.
Last I checked (this was years ago) the way to deal with this is to wrap it in a sub-message (which are actually nullable), which is just kinda gross.