Patent GB-190710431-A (GB)
Title: Improvements relating to Single Line Intercommunication Telephone Systems.
Inventor: BIRNBAUM ISIDORE BERNARD
Filed: 1907-05-04
Granted: 1907-10-24
Abstract: 10,431. Birnbaum, I. B., and White, H. G. May 4. Telephone systems.-In intercommunication telephone systems, a buzzer is put into operation when the receiver is removed from the hook, thus causing a noise to be made in the receiver so as to prevent overhearing, the arrangement being such that the buzzer stops operating when speaking connexions have been established. In order to reduce the noise made by the buzzer in the receiver, a form of induction coil shown in Fig. 2 may be used. The cores b of the main buzzer coils are connected to a thin yoke c, and have screw extensions d, adjustably supporting a second yoke f arranged within a coil g. Variations of flux in the coils a produce similar variations in the coil g, which is connected in shunt with the receiver. If subscriber I desires to call subscriber III, the selecting-lever o' is placed on the contact 3 and the ringing key y' depressed, thus closing a circuit from the battery m<1> through the key y<1>, bell u<1>, line x, bell u<3>, contacts w<3>, s<3>, t<3>, line 3, and selector o', to the battery m<1>. The attraction of the bell armature v<3> opens the contact w<3>, thus breaking the line circuit intermittently so that the two bells u<1>, u<3> are sounded synchronously. The sounding of the bell u<l> indicates to the calling subscriber that the called subscriber is disengaged. The removal of the receiver from its hook at station I completes a local circuit from the battery m<1 >through the buzzer coil a<1> and microphone k<1> in parallel, and the contacts n<1>, r<1>, q<1>, p<1> back to the battery. The vibration of the armature q<1> produces an intermittent current in the coil g<1>, which shunts the receiver h<1>. In the same way, the removal of the receiver at station III causes the actuation of the buzzer a<3>. The operation of the buzzer at each station continues until the called subscriber places his selector o<3> on the contact of the calling subscriber, thus completing a circuit from the battery m<1> through the buzzer a' and microphone k<1> in parallel, contact n', receiver h', line 1, selector o<3>, battery m<3>, buzzer a' and microphone k<3>, contact n<3>, receiver h<3>, line 3, and selector o' back to the battery m'. This circuit does not include the back contacts of the armatures q<1>, q<3>, so that the armatures of the buzzers are permanently attracted, and the noise in the receivers ceases. Conversation can then take place. If a third subscriber attempts to tap the line, as soon as he removes his receiver his buzzer is operated and can only be stopped by the establishment of speaking connexion with some disengaged station. Several alternative arrangements of the buzzer are described, in one of which the completion of the speaking connexions between the two stations connects the batteries of the two stations in opposition as regards the buzzer, which is thus rendered inoperative. The buzzer may also be electrically or mechanically controlled by an electro-magnet in the line circuit. The buzzer coils may form the primary of the usual induction coil, and may be arranged in series or in shunt with the microphone.