Some distributors used numeric codes. "39" could refer to the year 1939, though the fourth book was actually published in the mid-1950s (typically 1955–1960). This is unlikely.
(transmitting speech via light beams) and a photoelectric alarm. Practical Tools the boys 39- fourth book of radio and electronics pdf
: While the educational theory remains sound, hobbyists should note that some specific vintage components mentioned in the book (like certain germanium transistors or 67.5V batteries) may be difficult to find today and might require modern equivalents. Alfred Powell Morgan: the Eternal Boy Turns 120 - ARRL Some distributors used numeric codes
A solar powerhouse and solar-powered radio receivers. (transmitting speech via light beams) and a photoelectric
While you are searching for the PDF, know that the physical first edition of The Boys' Fourth Book of Radio and Electronics (with the original red cloth cover and dust jacket) sells for between $75 and $200 on AbeBooks or eBay. The reason? The diagrams are large, fold-out schematics that are difficult to scan.