It has been said that in the Internet age, if information isn’t digitised then it might as well not exist.  My kids certainly think that if it can’t be found by Google then it’s not worth bothering.  One thing that this removes is information on many of the dead technologies that time has moved on from and along with the technologies, also lost are the techniques that optimised them.  As an engineer, I find these old technologies and how they were tuned fascinating .. so from time to time I buy old books that detail the latest technology of the day.  One of these is Stubbs electrical encylopedia from (I think) the late 1930’s.  I was flicking through it a couple of days ago, and happened on an article about Mercury Arc Rectifiers.  These were the way that Direct Current supplies were provided for applications like railways at the time .  Now, you can find details of these devices in things like Wikipedia (e.g. here).  But, you don’t get to see the way it was really optimised – as a comparison, see an article here – from an encyclopedia about 25 years after mine!

Some of the examples in this book are clever, but some are frankly scary.  For example, one of the tricky things about mercury arc rectifiers is having vacuum proof seals through cabinets.  How might that be solved? – how about an asbestos compression washer that is soaked in mercury?  I jest not :-

‘c’ is the asbestos washer, ‘g’ the mercury, and ‘f’ the mercury reservoir.  Can you imagine being asked to clean that up?  The actual design is quite clever though – any leak simply results in a small amount of mercury entering the chamber .. and there is already quite a lot of it in there, so it does no damage at all.  But, I can see that getting scored a D- if it was posited as a solution now.

Mind you, it doesn’t sound like this was the only issue with these devices – for example, in the steel chamber versions the vacuum pump use to emit tiny amounts of mercury vapour – and over time that makes clean-up a major issue. And the smaller all glass units used to have breakage issues if not installed very carefully – and that suddenly meant a kilo or more of mercury on the loose … as a reminder on how dangerous mercury is, a compact fluorescent bulb has 1-3mg of mercury in, and if broken it is advised that a room is vacated for half an hour .. these devices has a million times more!

Mind you, the glass devices did look a lot cooler than modern semiconductor devices – they’d be right at home in a mad scientist lair – dig that mercury vapour glow :-

Here’s a close-up of the glass type used above scanned from my encylopedia :-

As an aside, there is loads more info on these devices at http://www.electricstuff.co.uk/mercarc.html.