Fleming Valve

Run the heater at its rated voltage and keep the plate supply at 24 V or less. Classic valve circuits use hundreds of volts and are not a beginner's project. The glass gets hot — let it cool before handling.

Fleming claims the space between a hot and a cold conductor "possesses a unilateral electric conductivity". The vacuum is the component — the metal parts only define it.

Put a low-voltage AC source across a moving-coil meter on a DC range. The needle sits near zero however large the signal is. This is the problem.

Use a small vacuum diode and its datasheet. Find heater, cathode and anode before wiring anything — a heater on the anode pins destroys the valve.

Bring the heater up to rated voltage and nothing else. A dull orange glow appears. That glow is the whole mechanism starting.

Put 12 V between anode and cathode, anode positive, through a 10 kΩ resistor. Record the current. It flows.

Swap the leads so the anode is negative. The current falls to essentially nothing. Same components, same voltage, no conduction.

With a cold filament nothing conducts either way. The one-way behaviour exists only while something is hot enough to emit — it is not a property of the metal.

Step the anode from 0 to 24 V and record current at each step. The curve rises, then flattens — beyond a point, every emitted charge is already being collected.

Run the heater a little hotter within its rating. The flat part sits higher: a hotter filament emits more. The cathode sets the ceiling, the anode only collects.

Replace the DC supply with a low-voltage AC source across the valve and resistor. The meter that read zero in step 3 now reads a real DC value.

Put a capacitor across the resistor. The pulsing output becomes steady — rectification plus smoothing is how every valve radio got its supply.

Swap in a silicon diode and repeat step 11. Same one-way behaviour, no heater, no warm-up, no glass. Note what it cost the valve to do the same job.

Try to make the output larger than the input. You cannot. A valve with two electrodes can only rectify — and that limit is what the next invention removed.

The patent. US 803,684, "Instrument for Converting Alternating Electric Currents into Continuous Currents", filed 19 April 1905 and granted 7 November 1905 to John Ambrose Fleming. The word valve is Fleming's own analogy — a one-way valve in a pipe — and it is why British usage still says valve where American usage says tube.

The device is an incandescent lamp with an extra electrode. That is not a figure of speech. In 1883 Edison, investigating why his lamps blackened, sealed a metal plate into a bulb beside the filament and found that current would flow from the hot filament to the plate but not back. He patented the observation, could not explain it, and put it aside; it became known as the Edison effect. Fleming had been a consultant to the Edison companies and knew the effect intimately, and he was separately working for Marconi on the problem of detecting weak wireless signals. His contribution was to recognise that a laboratory curiosity in lamp manufacture was exactly the missing component in radio. It is a clean example of an invention that is not a new phenomenon but a new use for a known one — and of why cross-domain experience is worth so much.

Why heat is doing the work. A metal at room temperature holds its free electrons; heat it enough and some escape the surface entirely, forming a cloud around it. A positively charged plate sweeps that cloud up, so current flows. Reverse the polarity and the cold plate is asked to supply electrons of its own — and having no cloud, it supplies almost none. The asymmetry is between hot and cold, not between two different materials, which is why step 8 is the decisive test: kill the heater and the component stops being a component. The flattening in step 9 is the second half of the same story — once the plate collects everything the filament is emitting, raising the voltage cannot produce current that does not exist.

What it could not do. Two electrodes give you a switch that is always in the same position: current either flows or does not, and nothing controls how much. Adding a third electrode — a grid between filament and plate, where a small voltage steers a large current — turns the valve into an amplifier, and that is Lee de Forest's Audion of 1906. The two men and their backers litigated over the relationship for years. Both devices are in this collection, and the honest summary is that de Forest's is unmistakably built on Fleming's, and that Fleming's is unmistakably built on Edison's.

The straight line to now. Rectification is still the first thing that happens inside almost every mains-powered device you own, and the silicon diode in step 13 performs Fleming's function with no heater, no vacuum and no warm-up. The valve's descendants ran radio, radar, television and the first electronic computers for fifty years, and were replaced by the transistor — also in this collection — which does the same jobs cold.