Some gorgeous illustrations from Telephony and Carrier Currents, an International Correspondence Schools textbook published in 1934.
I love this style of drawing. It has a hard practicality, and also a strangely haunting prettiness, right at the intersection of engineering and art.
Shout out @metkis for your patent illustration post earlier, which reminded me to dig out this book. 
@th
Spectacular! It must have taken weeks at least to make these engravings.
@th @cassvs @metkis this is not "somewhat silly idea" but real idea behind so called radio-fax: https://en.wikipedia.org/wiki/Radiofax . It is still used to transmit meteo maps over short waves. As well in ham radio world. My first serious program written for Amiga was just for transmitting pictures over radio using this idea.
@cassvs @metkis engravings of mechanical systems also often have a wonderful quality of depth and detail