I've come across another excellent amateur radio book from 1922. The book details all manner of circuits and apparatus including the variometer shown to the left. The variometer was a couple inductance device that worked by rotating one 'honeycomb' coil within another. When the coils were at right angles, very little coupling resulted and when they shared a parallel winding axis, the greatest inductive coupling was obtained.
The book also contains an inspiring discussion of the value of amateur radio to students. It's excerpted in the two pages below: