548 BELL SYSTEM TECHNICAL JOURNAL 



would otherwise occur in the confined air space between diaphragm 

 and cap. The contour of the ear piece, resembHng that of the famiUar 

 telephone receiver, permits the listener to readily center the receiver 

 on his ear. 



In order that an individual part of any telephone circuit may be 

 compared with the corresponding element of the master reference 

 system, the system is arranged so that the reference transmitter, 

 reference line or reference receiver, may be replaced by the corre- 

 sponding part to be rated. 



Calibration of Master Reference System 



To specify adequately the performance of the master reference 

 system, in terms of definite physical quantities, apparatus is associated 

 with the system for making electroacoustic calibrations of the reference 

 transmitter and reference receiver, and electrical calibrations of the 

 circuits. In general, the method employed in these calibrations is to 

 adjust the setting of an attenuator to obtain a deflection on the 

 galvanometer of a vacuum tube voltmeter, equal to the deflection 

 produced when measuring the output of the element under calibration. 

 This avoids the necessity for an absolute calibration of the measuring 

 device. 



The source of alternating currents, used for calibrating purposes, 

 is an oscillator, capable of producing currents with a harmonic output 

 usually less than 3 per cent of the fundamental. 



The measuring equipment used for making the above calibrations 

 consists of a two-stage vacuum tube voltmeter in conjunction with a 

 tuned circuit connected across its input. By means of this tuned 

 circuit, harmonics of the fundamental frequency to be measured are 

 attenuated by at least 20 db. 



The calibration of the condenser transmitter is made with a thermo- 

 phone, the gold leaf thermal elements being shown in Fig. 6. In 

 Fig. 5 is shown a cross-section of a condenser transmitter and a thermo- 

 phone. Petrolatum is used to form a seal between the instrument 

 and the thermophone block. The air in this small enclosed chamber, 

 formed by the walls and diaphragm of the condenser transmitter and 

 the thermophone block, is replaced by hydrogen. Since the velocity 

 of sound in hydrogen is approximately four times that in air, the 

 frequency range over which measurements may be made before 

 standing wave effects are experienced is extended by the use of this 

 gas. In addition, the efficiency of the thermophone is increased, the 

 constant of diff'usivity for hydrogen being greater than that for air. 

 Both alternating and direct currents are passed through the gold leaf 



