I was eight years old when my gran bought me the Meccano Elektrikit. After building all the motors, buzzers, bells, and other projects in the manual (including the fantastic, fully functional Telegraph Receiver with Bell and Morse Key, and the brilliant Electric Shock Machine!), I began looking for still more practical applications. I was almost eleven by the time I'd decided to drill a hole through my bedroom floor into the downstairs kitchen ceiling, and push through a pair of wires. These then became a simple serial circuit, comprising a battery and small lamp upstairs, and a switch downstairs.
Now at last I could play Mungo Jerry's In The Summertime, or Hotlegs' Neanderthal Man, at a decent volume on the Dansette. And when dinner was ready, mum - rather than having to come upstairs and bang on the bedroom door - could use her kitchen switch to let me know. Provided, of course, I just happened to be looking directly at the lamp at that precise moment (it wasn't much bigger or brighter than a single fairy light).
More often I'd be freaking out, kneeling eyes closed with a 12½" Meccano girder in each hand, thrashing the bed; the frustrated drummer, performing for his myriad adoring fans. Poor mum would patiently tap her switch until eventually the song ended, and I would notice the signal. Clearly this situation necessitated the introduction of a second channel of communication, pointing in the opposite direction, to let her know when I'd got the message.
Before pushing through another pair of wires to support this "back channel", I sketched out the full design - wires, switches, batteries and bulbs - on to my first circuit diagram. On inspection it struck me that here were two copies of the original circuit, behaving completely independently of each other; they should still work correctly if electrically joined at any single point. And, should that point happen to be one of the wires travelling through the floorboards, why then I could physically remove one of those four wires, and get by with just three.
Wait just another minute though. Suppose that common point was the '-' terminal of each of my 4½V batteries. Call that point "zero Volts". Then the '+' terminals of the batteries would both be at the same 4½V level above this. In other words, there's no voltage difference between them; were I also to connect the '+' terminals of my batteries together, no current would flow across the join. But adding this wire meant that I'd have just one battery powering the whole system, instead of two. It's fun discovering things like that for yourself!
A string of enhancements soon followed. Two more series bulbs were added, guarding against filament failure and reassuring the sender that a message was in fact being transmitted. When I discovered diodes, the battery was replaced by the 13V AC output of my Hornby model railway transformer, and the number of connecting wires reduced to two. And then to one, when I co-opted the household mains Earth wire for the return path: yes, I guess I was a preteen criminal.
Next time: the unique sound-to-light system of Blak Ice Disco.
Dansette image courtesy of / © The British Library Board.