The effect of a medium over the telephone reading a spectator's mind was developed by Bill Bowman. His method was described by Rufus Steele (who credited him as “Bill Bournau”) in Card Tricks You Will Do, 1928, p. 22. Bowman's method was a checklist of criteria on the medium's part to ascertain the selected card, such as whether the magician is standing near the spectator, whether the spectator introduced herself, whether the card was written down, etc.
The silent coding method of the medium listing off items until the magician says, “Hello?” on the correct one was first published by Howard Savage in The Sphinx, Vol. 28 No. 4, June 1929, p. 136. Savage's code was employed in a book test using a booklet of poetry with about twenty pages. The page number was transmitted, so that the medium could refer to a duplicate booklet and identify the poem selected. The application of his code to playing cards seems to have been made by William McCaffrey; see John N. Hilliard's Greater Magic, 1938, p. 566. In The Sphinx, Vol. 41 No. 3, May 1942, p. 49, J. G. Thompson and Charles Nyquist survey the field of telephone test methods in “Thanks to A. G. Bell”.
See also Telephone Codes.