I work in a Heritage Railway, so still use a lot of old stuff, main problem is heath and safety, as many items designed well before we worried about such things.
But also instructions like get your spokeshave, nothing about using a muti-tool. And today you can't get town gas, (coal gas) and not sure what the rules and regulations are about gas bags on the roof.
Clearly ohm's law has not changed, but even modern books get it wrong, I was looking at the wiring diagram for a fluorescent lamp in a modern 'A' level Physics book, and the ballast was missing.
So the diagram for a regulator.
It states
When the above regulators are overhauled in BREL the battery charge contactor (CTR) and the associated cut-in relay (CIR) and regulator relay (REL) are being replaced by a battery charge diode (BCD). This alteration is detailed in Engineering Instruction MD 328 issued to BREL.
but the instructions are not really good enough to repair and set it up. It has been long a question on how to fit lights in a heritage railway carriage, at the moment we use batteries and remove batteries for charging.
Today maximum speed permitted is 15 MPH, and it seems unlikely the old dynamos would work at such a slow speed, likely best option is solar panels on roof, but that is hardly heritage.
So what good are the old books? Even some thing as simple as an RB108 regulator used with so many cars, although I do have the Lucas workshop manual with all the settings, there are no spare parts, so a cut out is replaced with a diode, can't see what has been done once cover replaced, but it means the books are not much good.