580 
10 
The time constant of the gage is defined as the product of the 
total capacitance in parallel with the gage and the input resistance of the 
amplifier into which the gage output is sent. It is necessary that piezo- 
electric gages have a large time constant compared to the duration of the 
pressure under study. If the time constant is not sufficiently large, the 
Signal will leak through the input resistance of the amplifier and will ac- 
quire a corresponding distortion; see pages 24 to 27, inclusive, for detailed 
calculations which provide practical criteria for the magnitude of the time 
constant needed. 
Care must be exercised in the selection of a cable for piezoelectric 
pickup elements. It was found in the Taylor Model Basin tests that most ca- 
bles, when subjected to an explosion, generate a signal of their own which 
has a low amplitude and long duration compared to the true signal. If the 
cable signal is not removed, measurements of momentum in the pressure field 
are particularly unreliable. A comparison of the signals produced by various 
cables is given in the section beginning on page 12. The cable finally se- 
lected (18) consists of an annealed copper tube with an outside diameter of 
1/8 inch and a wall thickness of 1/32 inch. The central conductor is a Num- 
ber 20 or Number 24 copper wire insulated with enamel and a double layer of 
glass fiber.* The space between the central conductor and the wall of the 
copper tube is filled with ceresin or paraffin. The wax prevents vibration 
of the central conductor inside the copper tube when the pressure pulse from 
an explosion strikes the cable. 
THE TMB RESISTANCE GAGE 
It was felt desirable to supplement the piezoelectric gages with a 
gage of the low-impedance type of approximately 100 ohms. Such a gage is 
particularly convenient if a very long line is required between the pickup 
element and the amplifier, or if a signal of long duration is to be studied. 
Since the impedance of the gage matches the surge impedance of the cable, 
there is no frequency distortion of the signal by the cable no matter how 
great its length. 
The principle upon which the resistance gage operates is that its 
ohmic resistance changes when the pickup element is subjected to pressure. 
This change in resistance affects the current in a ballast circuit. The cor- 
responding variation in voltage across part of the ballast resistance is re- 
corded by the cathode-ray oscillograph. It is evident that insofar as the 
pickup element is concerned, there is no limitation on the maximum duration 
ie This wire is manufactured by the Anaconda Wire and Cable Company under the trade name of DVE wire. 
"DVE" wire signifies double vitrotex enameled. 
