Danny and the Multiple Alarm Clock (Part II)
Before we start, I have to say that I've never actually seen inside this mechanism, for obviously there must have been some way of opening it up, as the lower clock would need access for winding. Nor have I seen any kind of Schematic Diagram, so this is how I think it must have worked.
All I know from overhearing references to the beast is that there were two clocks involved, also "knitting needles", and that it was plugged into the mains. From that I have conjectured this design which I'm now about to present to you.
If there be anybody within earshot who can remember anything about it, please come forward and tell us. It would undoubtedly have had a name, probably that of the F/Sgt who'd designed it, but if tnat were so I've forgotten it.
You may think I sound remarkably incurious about this new toy, but all I was concerned with was how our Assistants worked it. It was simple enough. When a solo JP took off, the Asst. took out two pins with the callsign, one for Local's desk display, the other went into the perspex top of the Alarm, in the segment indicated by the pointer of the clock minute hand.
It might be that several aircraft took-off during that same minute, in which case they were added to those already there. I think there were eight holes along each degree segment, and even if they were all full, you could put any extras into the next one - the chap would get his call only a minute late.
Now comes the crafty bit. Below the first clock was a second one. And, immediately below the top perspex, stretching down the mid-point of every one minute sector, was fixed a pair of (colloquially known as) "knitting needles". These were pairs of small brass or copper rods of some 1-2mm section, normally set parallel, very closely (0.5mm ?) apart, but not actually touching. A Mapping pin could pass through the hole above, and make good electrical contact between them.
The rest was easy. you took the glass off No.2 Clock (set 40 mins after No.1), rigged a tiny metal contact to brush lightly over the tip of the minute hand and went to work with thin flex and soldering iron to set up a low voltage DC circuit between minute hand, spring contact, rod to pin to rod, on to the buzzer/squeaker/bulb, and back.
Then do it all again 59 times, each time wiring to the next minute's rod (obviously at the base, for there you'd have about 30mm (the width of your segment) to work in, whereas, at the top end (eight inches up) there would only be 10mm. Add a transformer/rectifier, plug-in and off you go.
A difficult point might be the fixing of the pairs of rods so that they were held the exact distance apart but still leaving sufficient air gap to ensure the necessary insulation. A tiny block of perspex, drilled with two holes (even in the worst case of, say, 2mm rods), could fit in 4.5mm overall for holes and still give room of 2.75mm at each side for fixing at the top of the segment. Should be a doddle for any instrument mechanic !
And all this is mere supposition - just my guess as to how it might have been be done (there must have been other ways - probably better !)
40 mins on, the No.2 clock hand would have worked round to the segment of the original pins; these bridged the rods gap, the circuit was closed, and the Alarm did whatever it was set up to do. For the rest of the minute it would buzz (or whatever) until the last Bloggs in the segment had been warned and his pin removed. IIRC, there was no follow up, clearly any Bloggs who ran out of fuel after such a timely reminder should clearly have been "chopped".
As for the Alarm Box I remember, the varnished woodwork was of high quality, it was bulky and very heavy, much too big to put on a control desk. So it had to go on a separate table, and as Local's Assistant sat by him at the desk, I suppose we had to be established for another Asst. to sit at this table and operate the device.
And that's really all that needs to be said about the Multiple Alarm Clock.
Anyone with anything to add ?
Good evening once again, chaps,
Danny42C.
Ingenious, these Chinese !