[Fig. 132.—Diagram showing the principle and construction of the telephone receiver.]

[Fig. 133.—The photophone.]

[Fig. 134.—Photophone receiving apparatus.]

[Fig. 135.—Photophone transmitting apparatus.]

[Fig. 136.—Powerful searchlight arranged to transmit speech over a beam of light.]

[Fig. 137.—The electric arc.]

[Fig. 138.—Circuit showing how a singing arc is arranged.]

[Fig. 139.—A logical form of wireless telephone which is impractical.]

[Fig. 140.—DeForest wireless telephone equipment.]

[Fig. 141.—Wireless telephone receiving apparatus (induction method).]