•Tltr. STF.THOPHONE," AN ELECTRICAL ^TF.TIIOSCOPE 543 



(iptTatiiii; ronditioiis wlu-n ihc output rereivt-rs art- f(|uippt'(l with 

 riit)hfr tubing as in Kij;. 7, and with the binaural i-ar-piens hild in 

 ihi- oars. '\'\\v peaks in this curve are due principally to the resonance 

 in the air columns of the rubber tubing, and correspond to the sim- 

 ilar |)eaks of Fig. 3 for the open bell stethoscope. In order to point 

 out the effect of the stethoscope attachment of the output recei\-er, 

 a second characteiistic is shown by a dotted cur\e which represents 

 conditions when an output receiver of the same type is held directK 

 against the ear. It is noted that the stethoscope attachment in- 

 creases the transmission between 150 and 500 cycles per second, 

 and damps the sharp resonant peak of the receiver. 



The overall characteristic of the stethophone as employed for 

 .iiiscultation is quite similar to that of the open bell stethoscope. 

 It is desirable that the body sounds as observed by the stethophone 

 shoulil appear the same as in the ordinary stethoscope, particuIarK- 

 in teaching work since the latter is used almost universally in regular 

 practice. If it were deemed desirable for special purposes to avoid 

 the tlistortion intnxluced by the stethoscope attachment, receivers 

 with small tips to insert in the ears could be used. For such an 

 arrangement, the overall characteristics could be further improved 

 by using damped recei\ers which would practically eliminate the 

 sharp peak of the dotted curse in Fig. 8. 



0. InsT.VLLATION for Te.\( IIING PtRPOSIiS 



When the stethophone is to be used for teaching purposes a perma- 

 nent wiring or distribution system should be installed with outlets 

 distributed among the seats of the amphitheatre or lecture room." 

 .A schematic diagram of such a system is shown in Fig. 9. A dis- 

 tributing pair of feeder wires, preferably shielded, is run between 

 alternate rows of seats below the floor casing, or suitably sheathed 

 to prevent damage. An outlet block "A" of six double contact jacks 

 is mounted on the back of each third seat of alternate rows. Thus, 

 one outlet block will supply si.\ seats, three in front and three in 

 back of the block. -Substantial jacks should be used throughout anrl 

 all receivers should be equipped with rugged plugs. In addition 

 to furnishing jack outlets among the seats, two or three multiple 

 nutlet blocks may be installed at the center of the amphitheatre as 

 shown at "B" for the use of guests or others on the floor of the amphi- 

 theatre. The output of the stethophone can be connected to the 

 distributing system of the amphitheatre at any one of these outlets. 

 Switch btjxes should be installed at various points as at "C" to facil- 



