I would think that his generator would be in the form of a dynamo (as opposed to an alternator). A dynamo makes use of a commutator (just like a dc motor) to switch the direction of current so that it doesn't alternate.
Dynamos were commonly used in cars for battery-charging purposes, until they eventually got replaced by alternators (this change being made possible by the advent of cheap solid-state rectifiers).
Initially electricity was only from batteries so it was natural that dynamos would be used to charge batteries or provide a DC supply.
I seem to remember the AC vs. DC battle in the US got quite dirty with one of the companies involved trying to claim AC was more dangerous because it was used in the electric chair (or was it the other way around?).
Some bits of the programme were quite basic but it was interesting seeing the big machines and their constituent parts.