•'354 SECTIONAL TRANSACTIONS.— A. 



(1) The source of sound consists of a telephone diaphragm set in vibration 

 by an oscillatory current, generated in a tuned circuit by means of a ther- 

 mionic valve. The circuit is so designed as to give an exceedingly pure sound, 

 its most efficient note being 500 vibrations per second. The note can be varied 

 in pitch by alteration of the capacity in the circuit. 



(2) The receiver consists of a high-resistance Wheatstone bridge circuit, 

 into which the oscillating current is supplied by a silent mercury key. The 

 telephone used for imparting the signal to the ear is placed across the bridge, 

 and the strength of the signal is varied by throwing the bridge out of balance 

 by a measured amount. 



A device for measuring ' paracusis ' is included in the receiver. A disturbing 

 sound of constant intensity, capable of reproduction in successive tests, is 

 superimposed on the telephone diaphragm, and the strength of the signal is 

 adjusted as before, until the latter is just audible. 



Suitable acuity scales are given by the bridge readings. 



ii. Standard Source of Sound. 



This instrument was designed by Captain Paris to give a sound whose 

 intensity can be measured in C.G.S. units. It consists of a Helmholtz resonator, 

 the base of which is formed by a telephone diaphragm. The diaphragm is 

 excited by an acoustic oscillator, and is of such pitch as to set the resonator 

 in resonant vibration. The sound emerging from the open orifice of the 

 resonator produces a measurable effect on a hot-wire microphone mounted in 

 the neck. Within limits, ohmic change in the hot wire is proportional to the 

 intensity of the sound. A knowledge of the constants of the hot wire, derived 

 from separate experiments, enables one to measure the intensity of the sound. 



iii. Aviplifier for Magnifyiyig Sounds such as Heart Beats. 

 This is a 4-valve resistance amplifier, in which only one transformer is em- 

 ployed, and is designed to reproduce the lower-pitch microphone currents with 

 as little distortion as possible. It is applicable to the examination of heart 

 sounds. 



(b) By Mr. F. E. Smith, F.R.S. — Apparatus for Testing Audition. 



The current ir a triode valve circuit is caused to oscillate at an audible 

 frequency, the value of the alternating current being directly measured by an 

 ammeter in the main oscillatory circuit. The frequency of the oscillation is 

 varied by varying the inductance, or capacity, or both, and the intensity of 

 the current is cojitrolled by varying the filament current. Included in the 

 plate oscillatory circuit is the primary coil of an air transformer, the secondary 

 of whicli is connected with a telephone, thermophone, or other receiver. By 

 varying the intensity of the oscillatory current, or by varying the mutual in- 

 ductance between the coils of the transformer, or by varying both, the note 

 from the receiver can be rendered inaudible. If the mutual inductance is 

 known, apparatus of this kind can be used as a standard of reference for 

 measurements of acuity. If the amplitude of vibration of, or the total energy 

 emitted from, the receiving mechanism is known, the apparatus can he used 

 for absolute measurement of acuity. To determine the amplitude of vibration 

 a thermophone method can be used, or a piezo-electric crystal can be employed. 



(c) By Dr. G. Wilkinson. — Worlang Model illustrating the pre- 

 sumed Resonating Mechanism of the Cochlea. 



The model consists of a brass box in two chambers (scala vestibule with 

 ductus cochlearis and scala tympani). A window closed by a rubber membrane 

 opens into each. To one of the membranes is attached a small wooden 

 plunger, the ' stapes.' The chambers are divided by a ' basilar membrane ' 

 formed of strands of fine phosphor-bronze flat wire stretched transversely, and 

 plastered over with fine paper saturated with formalised gelatine. The tension 

 on the threads has been regulated by suspending from them a series of weights, 

 graduated according to the formula 



n=i\/^ 

 21 db 



