A. B. Wood 177 
Fig. 10.14. Scanning mechanism. 
oscillograph, causing the spot to move upand down on the screen in synchronism 
and in phase with the transducer point in the water. The received signal is used 
to modulate the brightness of the cathode ray spot after suitable amplification by 
a preamplifier and a three-stage tuned amplifier. The hand-operated brightness 
control of the CRO is adjusted until the spot is barely visible at the surface and 
bottom turning points whenno soundis being transmitted, When the transmitter is 
switched on, the cathode-ray spot moving up and down on a vertical line on the 
screen appears as a series of irregularly spaced bright dots and/or dashes due 
to the fluctuations of sound intensity as the scanning transducer moves between 
the surface and the bottom of the water. If now in addition the trolley carrying 
the receiver moves along the tank, the arrangement of bright and dark spots on 
the CRO changes according to the position of the receiver. Consequently, ifa 
record is made by a film moving slowly in a direction at right angles to the line 
of dots on the CRO screen, while the trolley moves from one end of the tank to 
the other, a continuous picture is recorded of the whole sound pattern ina 
vertical section along the tank. It is important, however, if a more or less uni- 
form density of pattern is required, to compensate for the general decrease of 
sound intensity with range, due to spreading and attenuation in the water and by 
reflection at the bottom. This is achieved automatically by means of resistances 
shunting the output of the receiver preamplifier, one of which is automatically 
varied by the moving trolley and the other preadjusted by hand according to the 
circumstances pertaining when a record is to be made. 
Using barium titanate point transducers, good records can be made by the 
method described above when the transmitting voltage is as low as one volt. 
The sound intensity in the water in this case is of course very low indeed com- 
pared with that which was used to produce the bottom records in Fig. 10.12. 
