A reader of my last article pointed out a bit of a mistake on my part in saying that we generally don’t create AC electromagnets. When I said that, I was thinking of strictly electromagnets in the basic sense of the word. While AC electromagnets are always going to suffer from magnetic hysteresis (which can be reduced through materials and engineering), the changing of the poles is often not an issue since the electromagnet may be attracting something that itself is not a magnet. So the object would be attracted the same to either a north or a south pole. A solenoid is an electromagnet that pulls a metal plunger into its center. Since the plunger is likely not magnetic itself, it’s not necessary for solenoids to be powered by DC.
I was actually on vacation two weeks ago and when I’m relaxing at home, I often find myself watching youtube videos about interesting older technologies or just computer/electronics engineering type stuff. I saw one video in particular last week that was very interesting. It was explaining a very old type of computer memory called delay line memory. Well, old to me, I was born in 1988. Delay line memory would’ve been commonly used in computers starting in the late 1940s. The video on youtube was only marginally able to explain how it worked so I won’t try here, but it raised an interesting point about where all of our solid-state, semiconductor digital components came from.
These days, if you want solid state memory (RAM, NVRAM, etc) it’s an almost trivial affair. I have a micro SD card sitting on my desk at home that’s smaller than a penny and can store 64GB of data and has read/write speeds in excess of 100mbps. This is achieved largely through our ability to create microscopic devices called floating-gate MOSFETs (Metal-Oxide-Semiconductor Field-Effect-Transistors). These devices work by essentially keeping a charge stored for extended periods of time and they can be manufactured at incredibly small sizes. That’s the power of semiconductors for you!
But before we had devices like that, we actually used what I’ve been discussing in the past few articles here: induction. Magnetic core memory was initially developed in the late 1940s and was the preferred computer RAM up until about 1975. Magnetic core memory works by using lots and lots of little toroids (metal rings) with wires running through and around them. Through induction, the toroids can be “set” as having a magnetic field, or not having one (1s and 0s). By assembling the toroids in a grid or cube, you can have a nice “compact” block of memory. The grid pattern allows for an organized method of reading and writing chunks of data to the memory.
Because core memory uses simple magnetics as the storage mechanism, it’s actually a form of NVRAM (non-volatile RAM) which means the data persists even when the device is powered down. Flash drives and SSDs are examples of NVRAM. Core memory is also largely unaffected by electromagnetic pulses and ionizing radiation. For this reason, core memory was used past the 1970s on spacecraft including the space shuttle. Flight data recorded to core memory survived the explosion of the Challenger shuttle. Modern memory used in spacecraft is generally “radiation hardened” using insulating substrates, bit correction, shielding, or redundancy. Magnetoresistive RAM (MRAM) is a newer technology that promises to be more radiation resistant.
I think this article ended up being kind of a 30,000-foot view of this topic, and I’ve had to drop in some new terms and descriptions without complete explanations (transistors, semiconductors, etc). I might use next week to elaborate on some of these subjects. As always if there’s a topic you’d like me to explore, drop me an email!