Servoscribe

    Trying to describe a sparkline for my employer in a single concise sentence earlier today I was reminded of the machines that were my charges in my first ever job. Servoscribe potentiometric chart recorders, I've not thought of one of those in at least fifteen years!
    So, dear reader, if you'll permit me to indulge in a little reminiscence as a tonic for girl fog and for wearying of the faded warblings of the trannier-than-thou, I'll continue on the subject of outmoded laboratory equipment.
    A Servoscribe can best be described not as a machine or a device, but as a contraption. An unlikely Heath Robinson-esque assemblage of  machinery, electronics and electromechanical devices, they were a uniquely 1950s understanding of the problem of accurately recording a voltage that might vary over time.
    An electric clock drives a roll of graph paper at a user selectable rate past a pen. The pen has a mechanical linkage to an electric motor and a potential divider - just like the volume control on your TV - and the motor's job is to match the position of the pen and the potential divider such that the voltage from the latter matches the voltage on the input terminals. Clear as mud? The simple version is that a pen moves across a sheet of paper as the input voltage changes, leaving a line on the paper to form a graph. Connect a sensor, for example a thermocouple, and you can plot the change in temperature over an hour, a day, a week or whatever you fancy.
    All those mechanics driving the pen made a Servoscribe a pretty complex device, but they were the easy part. They were built to last, machined not from the chocolate they made cars from in those days but from good quality metal. The Servoscribe's party piece was its chief weakness, the chopper-stabilised DC motor amplifier featuring a mechanical switch in the form of a vibrating reed inside a glass tube.
    Modern electronics is easy. All the problems have been solved. If you need an accurate DC amplifier, you just order one. It's called an op-amp, and it's about the size of an earring and it'll cost you about twenty pence. But back in the 1950s they didn't have the advantage of such luxuries so they had to think laterally. They couldn't make good DC amplifiers but they could make good AC amplifiers. So what if they turned DC into AC, amplified it with an AC amplifier, then turned it back to DC? Simple, no?
     So they gave us the chopper-stabilised amplifier. A reed switch vibrating fifty times a second chops up the DC into AC for the amplifier. That's right, fifty times a second. By the time I got my hands on Servoscribes they'd been out of production for nearly twenty years, and the reed switches in most of mine had been vibrating for a lot longer than that. Those reed switches, and their care and nurturing, became the bane of my life.
    A Servoscribe was a cool piece of kit in its day. It was an interesting piece of kit as a curiosity in my day. Hell, I wouldn't mind one today for old time's sake, there must be something I can plot meaningless graphs of! But science has moved on, events can be measured in so much more detail these days with a computerised data recorder and the kind of things they used Servoscribes for in the 1950s are now so passé as to be taught to schoolchildren. Some of them have even been thoroughly debunked, replaced with far more interesting and clever science that does a better job of explaining the greater breadth of understanding that the intervening decades have given us.
    It's been an interesting trip down memory lane, thinking about the Servoscribe. But like all scientific dinosaurs that have had their day, there's no point in hanging on to one just because it once did a very good job. However much you may like the warm feeling that having one in the lab gives you.