DEVELOPMENT OF A HANDSET FOR TELEPHONE STATIONS 247 



Stand. The handset transmitter, accordingly, is not only used in a 

 wider range of positions but is also moved much more frequently with 

 the result that, in general, much wider variations are experienced in its 

 transmission characteristics and resistance. If the transmitter is not 

 suitably designed these variations may be sufficiently large to render 

 the transmission of a handset unsatisfactory, and to interfere with the 

 operation of associated relays and other signalling apparatus. 



The more severe use to which the handset is subjected also has a 

 tendency to materially accelerate changes in the contact surfaces of the 

 carbon and the electrodes. These changes are evidenced principally 

 by increased carbon noise and resistance which affect both transmis- 

 sion and signalling and appreciably shorten the useful life of the trans- 

 mitter unless proper design measures are taken to reduce these aging 

 effects. 



Differences in Structure 



It is undesirable in any telephone set, designed for use by the 

 general public, to require the user to perform any switching operation 

 in changing from talking to listening. For this reason, practically all 

 commercial telephone sets are of the so-called "invariable" type, in 

 which the transmitter and receiver are at all times connected to the 

 line while the set is being used.^ Part of the output of the transmitter, 

 therefore, is transmitted to the receiver of the local telephone set. 

 This electrical connection between the transmitter and receiver of the 

 same set is known as the "sidetone" path, and the resulting sound in 

 the receiver as "sidetone." In addition to this electrical coupling 

 between the instruments, there is acoustical coupling through the air 

 and in the case of the handset mechanical coupling through the handle. 

 If the amplification afforded by the transmitter is greater than the 

 aggregate losses in the sidetone path, the receiver, the air and the 

 handle, sustained oscillation or "howling" may be set up. Any such 

 condition, is, of course, not only unpleasant but fatal to transmission. 

 It has been found necessary, in fact, to keep w^ell below the howling 

 point to avoid serious transmission impairment due to transient 

 oscillations. 



In the case of the deskstand, the sidetone path and the air path pro- 

 vide the only coupling between the instruments and the losses are of 

 sufficient magnitude that howling does not occur even with the most 

 efficient transmitters and receivers. In the case of the handset, how- 

 ever, the handle may add appreciably to the coupling. Without proper 

 design of the handle, instruments, and means for mounting the instru- 



2 "Transmission Circuits for Telephonic Communication," K. S. Johnson, U. Van 

 Nostrand Co., 4th printing, p. 105. 



