582 
12 
series with a 100-ohm ballast resistor and four 6-volt storage batteries is 
subjected to a pressure of 3000 pounds per square inch, the voltage across 
the element changes by 5 millivolts. Furthermore, the gages are fragile com- 
pared to the piezoelectric elements and they seldom withstand more than half 
a dozen explosions. 
When long lines and signals of long duration are encountered, it is 
desirable to use the resistance element as a check on the piezoelectric gages. 
The resistance gage may also be useful in determining whether or not cable 
signal has been eliminated from a piezoelectric gage; this is particularly 
important in the case of explosions of large charges. Long lines are then 
used so that the operating personnel and the amplifiers will be at a safe 
distance. Thus successive parts of the cable are subjected to the pressure 
wave over a relatively long period of time; i.e., the time required for sound 
in water to traverse the cable length. If any signal resulted from this pres- 
sure on the cable, the tail of the true pressure curve would be obscured. The 
resistance element should show no cable signal under these circumstances. 
This is important particularly if the momentum as well as the peak pressure 
in the pressure pulse is being investigated. 
At the time of writing, the resistance element is not sufficiently 
rugged to be used as a field gage. 
The sensitivity of the glass elements to pressure changes varies 
from 1.4 to 1.8 x 1077 SS ee It is interesting to note that 
the compressibilities of various types of glass range from about 1 to 2 x 10° 
wit wee OV Alea reer 8 
pounds per square inch’ 
7 
PART 3. CABLES AND RECORDING CHANNELS 
TYPES OF DISTORTION OF A GAGE SIGNAL 
It has been mentioned in Part 2 that a long cable will act as a 
transmission line. Thus a signal propagated by such a cable will at a given 
instant possess different phases at different points along the line. This 
may give rise to distortion of the gage signal. Moreover, there are other 
causes of signal distortion, such as mechanical disturbance of the cable, in- 
sufficiently large time constant of the circuit, and inadequate frequency 
response of the amplifiers. These will be discussed in turn. 
DISTORTIONS DUE TO THE TRANSMISSION LINE 
The usual equations (19) describing a transmission line are in- 
cluded here for the reader's convenience. 
