Foley Telephone Mouthpiece 201 



them are almost pure. The tubes used in the experiments were so sensi- 

 tive that it was only necessary to heat the wire to a dull cherry red 

 temperature to make them sing in a smooth, loud tone. The especial 

 advantage of the electric heating even over any open flame was that 

 it gave an even temperature that did not seem to injure the tube by 

 congealing the glass. 



As would be expected, it was found that a proper balancing of the 

 capacity and inductances in the telephone circuits gave the best results. 

 Two common telephone coils at L and L' against a 1/3 m. f. capacity at 

 C seemed to give the best results of any arrangements tried. 



Considerable experimenting with different telephone "hook-ups" was 

 done before it was finally decided that the arrangement in figure 2 would 

 be best for our purpose. The theory for the arrangement is as fol- 

 lows: With switch S on contact point 1 the circuit for transmitter A 

 is complete. Any steady currents passing through transmitter A will 

 pass unimpeded through the inductance L, but intermittent or A. C. 

 currents would suffer considerable impedence. The intermittent or A. C. 

 currents set up by the transmitter diaphragm will, however, readily 

 discharge through the condenser and the receiver set. By this arrange- 

 ment no direct currents flowing in the circuits ever disturb the receiv- 

 ing set T, but practically the whole energy of the A. C. currents crea- 

 ted by the vibrations of the transmitter diaphragm will discharge through 

 the condenser. You will notice that both the circuits A and A' are just 

 alike. 



In taking data, one of the transmitters, assume in this case A', 

 was used as a reference transmitter. The tube K' was set singing and 

 resistance was "plugged in" in R' until the sound was just audible in 

 the receivers T, S being set on contact point 2. With this adjustment 

 made the switch S was set on contact point 1 and the tube K set to 

 singing. Resistance was now "plugged in" in R until no difference 

 could be noticed in the intensity of the tone when the switch S was 

 rapidly thrown from point 1 to point 2. (The tubes K, K' were of 

 practically the same pitch and as nearly the same size as it was possi- 

 ble to make them.) No change was made in the resistance R' for any 

 set of readings after once adjusted for minimum audibility. The first 

 reading on each horn studied was made with the mouth of tube K just 

 flush with the end of the horn on the transmitter A. Readings were 

 then taken as the tube was moved back to various distances from the 

 mouth of the horn. By placing different types of horns on the trans- 

 mitter comparisons could be made of the relative intensities of the tones 

 transmitted at any distance. In this research comparisons were made 

 for no horn, ordinary flared horn, and the Foley Telephone Mouthpiece. 

 The observations were made in an especially arranged, sound-proof 

 room; the walls, floor, and ceiling of which were covered with felt about 

 one inch thick. Long strips of this same felt about one foot wide were 

 also cut and hung edge-wise along the walls and ceiling to break up 

 any reflections that might occur. The singing tubes K, K' were set at 

 opposite ends of the room, a distance of about twenty feet, and the 

 receiving apparatus was moved out into the adjoining hall. Observa- 



