Intercommunicating Telephones.
—This form of telephone is used over short distances such as from room to room in buildings or for connecting the house with the stable, garage, etc. It is complete, in that it possesses the same features as any other telephone but the signal is an electric call-bell instead of the polarized electric bell used in commercial telephone service.
Any telephone is made to perform two functions: (1) that of a signal with which to call attention; and (2) the apparatus required to transmit spoken words. In the intercommunicating telephone or interphone, the signal is made like any call-bell and parts are similar to those described under electric signals. The bell-ringing mechanism is included in the box with the transmitting apparatus and the signal is made by pressing a push button. It is not suitable for connecting with public telephones. Telephone companies, as a rule, do not permit connection with their lines any apparatus which they do not control.
The interphone of Fig. 237 shows the instrument complete except the battery. This form of instrument is inexpensive, easy to put in, simple to operate and supplies a most excellent means of intercommunication. Complete directions for installation are supplied with the phones by the manufacturers.
Fig. 237.—The intercommunicating telephone.