I remember when microphones were physically dangerous. It mentions carbon microphones with the main supply voltage going through them: http://www.douglas-self.com/MUSEUM/COMMS/mechamp/mechamp.htm.... But even into the 80s, microphones plugged into portable vacuum tube amps sometimes ended up with the plate voltage (200+ volts) on them and shocked people when they touched them. Stage performers usually brushed a mic quickly with a finger before putting their lips close to it, to make sure it wasn't electrically hot.
I wonder if it resulted in people being more careful what they said near a microphone too.
Oh, mics are still dangerous. I play a lot of restaurants where you get odd hums... one way to "deal" with that is to remove the safety ground on your guitar amp.. of course, there is then a possibility that there is a difference in potential between yourself as grounded through the amp and yourself as grounded through a microphone... you don't need a carbon element or a tube amp to get badly shocked that way. I and a lot of my dirtbag musician friends still will touch a mic before using it.
I remember pneumatic tubes. Only a few years ago, Costco was still using them to deliver checks/cash to their tills. I cannot say exactly when, but well into the 2000s.
I was amazed / impressed to read that some of the pneumatic tube systems in cities were operating for over 100 years. That's a long time for any given technology.
I appreciated the bit about pneumatic tubes. For whatever reason, I have been itching lately to find technical details on modern packet-switched pneumatic tube networks [1]. They're still used in hospitals apparently.
I wonder if it resulted in people being more careful what they said near a microphone too.