How receivers with tappers worked

Okay, I think I’ve figured out how the very first receivers worked.  The really clever part is how the tappers knew when to tap.  Here is a diagram from 1899.  It’s what Professor Green was using at Notre Dame, so it derives from Marconi, by way of popular magazines (Marconi himself being secretive about his current equipment).  The magazine that printed this diagram was American Electrician, hence the ‘AMER. ELEC.’ caption.

The heart of the system is the ‘coherer’, item ‘C’ in the diagram.  This is not a detector, exactly, in the sense that Fleming’s ‘diode’ (or De Forest’s ‘Audion’) is.  We Radio Shack kit veterans will remember using tiny solid-state diodes to hear AM radio.  As far as I understand it, the coherer doesn’t do that.  If you put headphones onto it, you wouldn’t hear radio.  What the device does is drop its resistance as soon as radio frequency energy, which is to say, induced current from the antenna, goes through it.  That’s all.  It’s a honey bee-sized tube of nickel filings, mostly, that cling to each other, or ‘cohere’, in response to that current.  Suddenly the coherer’s resistance falls to tens of ohms, having been a matter of megohms before.  They will stay cohered even after current from the antenna ceases.  An automatic tapper physically jars them loose again, rendering the coherer into a high-resistance device again.  It’s a cumbersome system, where the tapper taps between each Morse code dot and dash.  Got it so far?  Keep your eye on that changing-resistance thought.

Look at the diagram.  Find the loop circuit containing the coherer, a battery, and three coils.  Notice that the antenna comes into the top of the loop, and there’s a ground connection out the bottom, too.  That battery isn’t strong enough by itself to make current flow around that loop.  That’s because the coherer resists too much.  But along comes a radio signal.  It hits the antenna, inducing current.  The current enters the loop and turns left, into the coherer.  The reason it doesn’t turn right is because of the impeding effect of the two ‘choke coils’, as they’re called (‘A’ and ‘A’).  The coherer coheres, resistance drops, and now the circuit activates.  The ground connection is to offer a place for the induced current to go.  In the absence of a relative vacuum, for so I think of voltage gradients, the current wouldn’t enter the system.

See coil ‘R’?  That’s an electromagnet.  Sometimes they’re called relays, other times solenoids.  It’s a coil with an iron core.  Activate it, and you close a metallic switch.  When you do — now follow the other loop, the one with the other battery in it — you get current that activates two other electromagnets:  one is ‘S’, that makes a mark on a moving paper tape, or else rings a buzzer, or just makes telegraph office clicks; the other is ‘V’, which squeezes the armature of the tapper against the coherer.

Here’s the clever part.  That tapper armature was already resting against the tube.  It’s springy.  If you bend it hard against the tube with all that current in the system — and then the current drops, because the radio signal stops coming through the antenna — the springy armature will give a little bounce against the side of the tube as its relay stops functioning.  Plink!  The particles are now decohered, ready for the next signal.

The light bulb at the coherer is to suppress arcing between the armature and the coherer (which would interfere with cohering and decohering).  I don’t know what the other light bulb is for.  It’s probably just to smooth current feeding the printer relay.

I think this is how it works.  Pretty clever!  It wouldn’t work very well if there were lots of people on the air.  It couldn’t have been very sensitive, either.  Professor Green tried to optimize it, by substituting an ammeter coil for the clunky buzzer coil he’d been using for his tapper.  If any engineer is reading this, and I’m misunderstanding something, please tell me!

This circuit diagram and attendant picture come from Jerome J. Green, “The Apparatus for Wireless Telegraphy,” American Electrician, July, 1899, pp. 344-6, reproduced at http://earlyradiohistory.us/1899nd.htm.