A NEW HIGH FREQUENCY ^'TONE -GENERATOR 
C. W. HEWLETT 
' ’ , ^ ABSTRACT 
Ordinary telephone receivers will respond satisfactorily to al- 
ternating currents at frequencies as high as ten thousand vibra- 
tions--, per second, but above this frequency so much of the 
electrical energy' is absorbed' 'ih hysteresis and eddy current loss 
that it becomes imp.raGticable to supply enough.. energy, to,. the re- 
ceiver to enable it to serve as a satisfactory source of sound'-waves. 
There are a number of iiitefesting applications for a generator 
of sound waves of frequency above ten thousand vibrations per 
second. .For instance the sensitiveness .of the ear for sounds of 
definite intensity has not been satisfactorily investigated between 
the frequency of ten thousand , and the so called upper audible 
limit. Another use is that such short waves will enable the phe- 
nomena of interference, diffraction and shadows to be demon- 
strated by the use of sound waves. There are other less obvpus 
applications, and on the whole it seems to be, worth while, to, try 
to develop a generator for high frequency sounds. 
If the generator is to be driven electrically it is obvious from the 
foregoing statement in regard to the ordinary telephone receiver 
that some method must be adopted in. which hysteresis and eddy 
currents are eliminated as far as. possible. With this in mind a 
generator of the following design has been constructed and used : 
Two flat pancake coils, each built of concentric coils with air 
spaces between are mounted facing each other with a thin alumh 
num diaphragm between. The coils and diaphragm are held in 
place by a framework of hard fiber or other insulating material. 
The method of connecting up the instrument and using it is 
shown below':" , ' 
A three element vacuum tube is used in a circuit forming an 
oscillation generator. and Lg are the coils- of the tone generator 
and form part of the inductance of a tuned circuit of which C 
forms the variable capacity. The capacity allows the alternat- 
ing current to pass through the coils without going through 
the 15 volt battery. This battery sends a direct current 
through the coils of the generator in such a way that 
