I've read his book and he talks about AC current frequencies of incredibly high magnitudes. How was this achieved? Surely he did not just spin his dynamo really, really fast.
His first experiments were with a unique new high frequency alternator . This was made up of a steel disk 30 inches in diameter with 384 poles like the teeth of a cog that had zigzag windings on them. This disk revolved within a fixed ring that had 384 inductor poles. When this machine was turned at 3000 rpm, it produced about 200 volts at 9600 to 10,000 Hz. Tesla was the first man to work with such a device . Later machines of somewhat different design produced up to 30,000 Hz.