426 
DR. W. S. TUCKER AND MR, E. T. PARIS ON 
The diagram (fig. 18) shows the nature of the polar curve of amplitudes. Experiments 
with sources of different pitches indicated that the higher the pitch the sharper the 
curvature in the region of the zero bearing. For highly directive apparatus, therefore, 
every advantage is to be gained by using big trumpets. 
The Hot-Wire Microphone gives an easy method of testing the resonance frequencies 
of trumpets, but the effect must not be complicated by using a resonant microphone 
of the type described above. In this case the trumpet itself may be used as the resonating 
cavity. A bare grid with an orifice of the type above described is fitted to the narrow 
end of the trumpet, as shown in fig. 19, and the amplifier is used with rectifier and 
reflecting galvanometer. The bridge method cannot be employed in this case as 
the open trumpet is subject to draughts, and there is constantly a movement of air 
in one direction or the other which would cause a change in resistance of the heated 
grid. 
The source of sound is the specially constructed siren referred to in § 4. As the pitch 
