From the 1941 edition of Grolier's THE BOOK OF KNOWLEDGE. I've read all volumes cover-to-cover and really enjoyed them.
AN AUTOMATIC CODER
Suppose you received the following message on a postcard: "ERNQGUROHGGREEHZPNEGBBA."
You would be greatly puzzled. You would wonder who could possibly be writing to you in Russian or Hungarian or Rumanian or whatever that outlandish-looking language is. And yet the fact is that the message is written in plain and simple English; only it is in code, produced by an automatic coder. Would you like to know how this coder works?
First of all print out Circles A and B, cut them out and paste them to a heavy piece of cardboard. Next cut out the two circles. Also, with a sharp knife cut out the slots formed by the two 4-sided figures in Circle B. Now run a pin through the center of Circle B and through the center of Circle A; fasten the pin to some sort of firm material like a drawing board or the side of a wooden box. Circle B must be on top.
You will note that when one of the slots in Circle B is so arranged that a letter is in the center of the slot, another letter will be revealed in the opposite slot. When one slot shows A the other will show N; when one shows B, the other shows O, and so on.
Now suppose that you wish to write "Come here now" in code. Turn the top card till one slot shows C, the first letter of the message. The other slot will show P. Set down P on a piece of paper as the first letter of the coded message. Now turn to letter O; the opposite letter will be B. Continuing in like fashion, M will give you Z and E will give R. The first word, therefore, corresponding to "Come" will be "PBZR." Continuing, "here" will be represented by "URER" and "now" by "ABJ." Run all three words together; this is done to make the decoding of the message more difficult for those not possessing the code. You will now have the message "PBZRURERABJ."
To decode the message, use the same method as before. That is, turn the top card of the automatic coder so that one of the slots shows P; the other will show C. Next turn the card so that one of the slots shows B; the other will show O. Continue with the rest of the message and you will have "COMEHERENOW," which you will recognize readily enough as "Come here now." Can you decode the message given at the beginning of this article?
Explain the code to one of your friends and have him make an automatic coder. You will find it great fun to correspond with your friend by postcard, using this code. Your family will be consumed with curiosity to know the meaning of these messages.