(225 ILCS 710/45.13) (from Ch. 96 1/2, par. 4266)
    Sec. 45.13. Every shaft, if exceeding 50 feet in depth shall be provided with an efficient means of interchanging distinct and definite signals between the lowest level and the top of the shaft and the various intermediate levels from which hoisting is being done. The signaling apparatus shall be a cord or wire actuating a knocker, bell or an electrical system.
(Source: Laws 1945, p. 1035.)