How did a 70 year old computer (Lyons electronic office I) use Mercury as memory? [2K (2048) 35-bit words (ultrasonic delay line memory based on tanks of mercury)]?

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How did a 70 year old computer (Lyons electronic office I) use Mercury as memory? [2K (2048) 35-bit words (ultrasonic delay line memory based on tanks of mercury)]?

In: Technology

Delay line memory works by sending a current through a loop where it is slowed down. If you want to write, you wait until the loop comes to you and inject a bit value. If you want to read, you wait until the loop comes around and read what’s on it. Mercury was one method used to delay the signal by having it turn into an ultrasonic pulse through a column of mercury, and then back into electric. One column could hold several pulses in flight at the same time.

Electricity moves very fast–nearly the speed of light. Sound, by comparison, moves at a snail’s pace.

This means you can set up some medium that can carry sound (they used mercury as this material). You have a transmitter on one end and a receiver on the other. The transmitter sends a bit of data by choosing one of two pulses (e.g. sending out a chirp for 1 and no chirp for 0, or a chirp at one frequency for 1 and another frequency for 0. The specifics aren’t important here, just that there are two distinguishable sounds that the transmitter can send). The receiver just has to be able to tell these two apart.

You then set this up as a loop. A transmitter sends the data as sound along the tank of mercury, then when the receiver gets that data it sends it back along a wire, repeating the process. The trick here is that the data takes a long time to move along the tank of mercury since sound moves so slow (compared to electricity). That means that after the transmitter has sent the first bit but before it has been received it can go ahead and send another bit, then the next, then the next, and so on. The longer the tank and the slower sound travels through it, the more data you can have “in flight” at once. That’s the data that is being stored. A major challenge here is that the sound will bounce around, so you have to design the system carefully to be able to tell the real data from the noise that arises in the mercury tank.

You can then interface with the electronic part of this system, where the signal is sent from the receiver back to the transmitter for the next trip through the mercury tank. A read would be carried out by waiting until the desired bits of memory reach the receiver and are sent back to the transmitter. A write would be carried out by waiting until the desired memory address reaches the end of tank, then whatever the receiver reads gets discarded and instead the new data is sent to the transmitter.