|Comptometer, photo by Daderot [CC0], from Wikimedia Commons|
As I indexed some World War II Registration cards for FamilySearch, I came across someone employed by the N. B. Davidson Comptometer Company in Indianapolis. I decided to Google "comptometer" to see what one was. I discovered an image similar to the one above and knew I'd seen these before and just assumed they were an early form of an adding machine. It pretty much was. Dorr E. Felt patented it in 1887, and it became the "first commercially successful key-driven mechanical calculator."1
At one time Comptometer Correspondence Instruction Courses existed.2 [The history of comptometer education, as recorded by Felt himself, appears in the same issue.]3
The publication Comptometer News appears to be a rich source of genealogical information (and photographs) if one's ancestor worked with the devices. Many issues from volumes one through ten (1926-1938) are available online at Internet Archive. The Felt & Tarrant Company produced the publication which appears to report news from comptometer schools across the nation, as well as a few international locations.
1 "Comptometer," Wikipedia (https://en.wikipedia.org/wiki/Comptometer : accessed 19 July 2018).↩
2 C. Vebeck, "Instruction Service Department," Comptometer News 3, no. 1 (January 1929), page 5; PDF, Jaap's Mechanical Calculators Page (https://www.jaapsch.net/mechcalc/pdf/compnews3-1.pdf : accessed 19 July 2018).↩
3 Dorr E. Felt, "The First Comptometer School," Comptometer News 3, no. 1 (January 1929), page 7; PDF, Jaap's Mechanical Calculators Page (https://www.jaapsch.net/mechcalc/pdf/compnews3-1.pdf : accessed 19 July 2018).↩