> The solid copper core is much thicker than my thumb. It's Conrad's eye of an engineer that is turned towards history that enabled him to spot and salvage this interesting relic that otherwise would certainly have been buried in a landfill.

I thought contractors just treated any copper they came across as an off-the-books bonus, not something that actually went to a landfill.

Related to this I recently found out about the origins of an old electrical distribution board under the stairs in my parents house, this was Southport, NW England. I knew it wasn old and not in use but was never removed.

Apparently the houses near the new tramway, installed about 1900, had an electricity feed direct from the tramway which passed the end of the street. I think it was possibly 600V DC from what I can see about tram power.

I’ve never researched it but having had a quick search can’t find anything specific to the houses being fed. I’ll bookmark for later as it’s an interesting step in the electrification of our towns. [1] is a link about the trams.

[1] http://www.petergould.co.uk/local_transport_history/fleetlis...

Part of the reasons while AC can be lethal even at lower voltages (110/220) is that the frequency (50/60 Hz) is near to that of the heartbeat and could create a sort of resonance stopping the heart.

Or at least, many years ago it was compulsory in mines and tunnel digging to have a sort of frequency elevator (at the time it was a generator (alternator) with output still at 220 V but with a very high number of Hertz, if I recall correctly 200-600, driven by an electric motor).

They looked more or less like:

https://bullseyeindustrialsales.com/giordano-colombo-c3ll-10...

From a given point in the tunnel (some 150 or 200 meters before the digging point) all cables/lightings were powered with this high frequency line, the idea being that if the cables were damaged in the works and a worker came accidentally in contact with a naked conductor he/she would get a (painful) shock but not risk life.

My understanding is that it's not anything to do with the resonance of a heartbeat or anything like that. The higher frequency ac will experience much more of a skin effect and not penetrate into the body as much or as easily. That means there will be less current that will pass through the organs and heart, but more through your skin and outer muscles. That'll mean it'll hurt more but be more likely to be survivable since you'll have more time to remove the person from the power source.

Most probably that is another part of the story, however it seems like there is the need of more current at higher frequencies:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/197300...

Look around page 40.

I was reporting what I was told at the time, anyway here is a chapter of an electricity textbook confirming (for what is worth) the added danger of low (50-60 Hz) frequency AC current:

https://www.allaboutcircuits.com/textbook/direct-current/chp...

Interesting, I wonder if the higher frequency ends up needing more current because the body can't respond as fast, acting like a filter because the muscles can only move so fast. Biology and electricity mix in interesting ways.

> the frequency (50/60 Hz) is near to that of the heartbeat

This doesn't smell right- if your heart's beating at even 5Hz, that's a whopping 300BPM.

I expressed myself incorrectly, I meant it is a multiple/resonance value, I cannot correct my previous post, unfortunately.

What is "semi continuous current"? Is that like pulsed DC?

My guess would be (mechanically) rectified ac. I'm not very knowledgeable about electrical machines, but I'd imagine that a generator with sinusoidal flux linkage and commutators would produce a waveform that looks like rectified single phase ac.

DC is only pulsed when used as a control medium. Maybe you're thinking of motor or actuator controllers that consume DC data?

Edison did not think DC was safer, but he had bo patent on AC, so marketed it as such. A rather famous swindle.

inspectopedia always has really interesting stuff for learning about how to inspect homes.

It’s a treasure from the “old web”. Information dense, simple markup, possibly updated by hand...

I miss the old web