This month’s Fessenden focus leads to a sidetrack. Several of Fessenden’s big innovations used high-frequency alternators instead of spark gaps. His first voice broadcast in 1906 used an alternator. The alternator deserves its own description before I dig into his most interesting use of it.
Before the vacuum tube there was only one way to form a smooth well-tuned sine wave at the high frequencies needed for radio. An alternator naturally makes a clean sine wave, and its speed can be controlled by old-fashioned governors, mature since the 1700s. The limitation was purely mechanical. If you try to turn a normal generator fast enough to reach 100 kilocycles, it will fly apart. The simplest alternator would need to turn at 6 million RPM!
The most common solution to this problem was invented by Alexanderson at GE. It was a classic example of the universal rule that real development always happens in the area of materials and methods. The Alexanderson alternator was much simpler than most generators. Most have the dynamic electromagnet on the rotor, needing a commutator to pick up the current. Alexanderson reverted to the original primitive idea of dynamos. The permanent magnet turns, so it’s uncomplicated and doesn’t need a commutator. What made Alexanderson special was highly sophisticated metallurgy and engineering, not the simple idea.
Here’s the simple experimental dynamo in action. A permanent magnet turns next to a coil. As the north and south poles approach and recede, current moves through the wire in opposite directions.
I demonstrated the basic idea last year using my primitive breadboard setup. I was moving a magnet across a solenoid coil in both directions, stirring up current in both directions.
Polistra and Happystar are showing an Alexanderson alternator powered by an electric motor. It runs much faster than the usual, but nowhere near 6 million RPM.
Alexanderson multiplied the speed of the magnet’s in and out without multiplying the speed of the shaft. He simply repeated the north-south transition many times around the edge of the simple rotor disk, with small coils on the unmoving armature to match the repeats of the magnetic field. My animation has 24 north-south pairs, represented again by red and blue areas on the rotor disk. Each pair is matched by a coil on the armature. The real thing was much denser.
As the rotor turns, each coil sends current in and out 24 times per revolution of the rotor. I’ve shown three of the coils, with the current arrow on the middle one. Because the patterns match, all of them are sending current in and out at the same time, so a parallel connection of the coils provides 24 times the current of just one.
Polistra's Mill 