of signal that may be studied. The 
low impedance of the pickup also 
serves to eliminate cable signal. 
An early form of a re- 
sistance gage, constructed at the 
Massachusetts Institute of Technol- 
ogy, consisted of a 250- or 500-ohm 
resistor* imbedded in a rubber 
sheath. 
average distance between the carbon 
The pressure changed the 
particles on the glass tube in the 
resistor, thus producing a change 
in resistance. This gage was found 
to be fairly sensitive when connect- 
ed to a 1500-ohm ballast resistance 
in series with a 45-volt battery; 
the voltage sensitivity was then 
about 5 x 1075 volts 
of the same order of magnitude as 
the sensitivity of a piezoelectric 
pickup. 
However, these resistor 
gages showed high hysteresis. The 
pounds per square inch’ 
581 
11 
mice ea. 
ro 
" 
Spot Weld—. Kovar Cylinder 
2 Kovar Rod 
a aaa Glass Tube 
Gloss 
\-mil Resistance Wire 
Figure 7 - Glass Resistance-Type Pickup 
In the upper diagram the gage is shown in its 
natural size. The element is made of 705 glass 
as manufactured by the Corning Glass Company. 
Kovar is used for the metal parts because it has 
the same coefficient of expansion as glass over 
a large range of temperatures. The Kovar rod is 
slipped into a glass tube, and the tube is put 
into a Kovar cylinder. 1-mil Advance wire is 
then wrapped about the glass tube and the two 
ends of the wire are spot-welded to the Kovar 
wire and cylinder, respectively. A small glass 
tube is then slipped over the end of the assem— 
bly at which the Advance wire is spot-welded 
to the rod. The entire wit is heated with an 
oxygen-hydrogen torch until the glass parts and 
the Kovar are fused into an integral unit. After 
the element is annealed it is ready to be assem 
bled on a cable. 
calibrations of some of them varied by as much as 20 to 50 per cent after the 
element had been exposed to a number of explosions. 
A gage was then developed at the Taylor Model Basin, which is elas- 
tic and which shows little hysteresis in the range of pressures encountered 
in underwater explosions. 
proximately 100 ohms of 1-mil Advance wire is imbedded; see Figure 7. 
This gage consists of a glass element in which ap- 
The 
ends of this fine wire are spot-welded to a cylinder and a rod of an alloy 
which has equal thermal expansion with the glass, known by the trade name 
of Kovar. 
7/32 inch. 
The overall dimensions of the glass element are about 1 inch by 
The Kovar cylinder and rod are set coaxially to prevent electri- 
cal leakage between them through the water. 
These elements can be calibrated 
in a static pressure chamber by measuring their resistance change with a 
Wheatstone bridge for a given applied pressure. 
tions and show little hysteresis. 
They possess linear calibra- 
This resistance gage has the disadvantage of very low sensitivity 
compared to the piezoelectric gages. 
When a 100-ohm resistance element in 
* As manufactured by the International Resistance Company. 
