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APPENDIX 2 
CONSTRUCTION OF TWO-PLY MINIATURE PIEZO-ELECTRIC TOURMALINE GAUGE 
AND CONNECTION TO SPECIAL CABLE. 
by R.V. Bundy and G. Hen 5 
‘le The two tourmaline discs, 4" in diameter and 1/16" thick are cleaned in 
benzene, and allowed to dry. Their resistances are then measured with a 
2500 volt megger and only crystals showing a resistance greater than 10,000 
megohms are accepted. 
20 A very thin layer of silver paste (Burnish silver paste No. 38 purchased 
from Messrs. Baker Platinum Ltd.) is applied with a spatula to each crystal face, 
and the discs are placed vertically with their edges resting in a V-block, 
contact between the silvered surfaces and the block itself being avoided. The 
assembly is now put in a pre-heated muffle furnace, the temperature of which at 
this stage must not be higher than 100°C. Its temperature is then raised 
slowly to 550°C, and is maintained for 30 minutes. The furnace is now switched 
off and allowed to cool to room temperature. The crystals are removed, 
re-tested with a 2500 volt megger and carefully lapped flat on a large tourmaline 
slab completely immersed in petrol. The operations described above take about 
34 hours to complete. 
3. The silvered discs are now tested for polarity and piezo-electric 
sensitivity, the positive faccs being lightly marked. The crystals are once 
more cleaned in benzene or petrol. 
he Each positive face receives a second and slightly thicker coat of 
Silver paste, and cach disc placed, positive side up, in separate porcelain 
crucibles and re-heated as described in paragraph 2 above, 
5. After this treatment, the resistance of each crystal is again checked, 
and the negative faces now receive their second coat of silver paste. On top 
of a steel pad $" in diameter and %" thick, resting on a specially constructed 
end plate of a jig, one of the discs is now fitted with its negative face up; 
a silver electrode cut to the appropriate shape from silver foil 0.0025" thick 
is placed in contact with this negative face, The second disc is now fitted 
in the jig with its negative face also in contact with the silver electrode 
and a further steel pad is inserted on top, The second end plate of the jig 
is bolted on to force the component parts of the gauge and the steel pads into 
alignment. The pressure required to force the erystals and silver electrode 
together is applied by means of screws in the end plates bearing directly on 
the steel pads, The gauge is now returned to the pre-heated furnace and the 
temperature raised slowly to 550°. 
6, The gauge unit now consists of two silvered tourmaline dises fused to 
the silver electrode located between them. The outer or positive faces of the 
gauge are burnished and the resistance between them and the centre electrode 
measured, If this resistance is low, it is usually found possible to restore 
the high value by cleaning the edge of the unit with an abrasive such as 
bakelised diamond dust, This last step is in any case advisable. Two copper 
electrodes 0.4" long x 0.12" broad are cut from annealed copper foil 0,00" 
thick, and spot soldered to the outer faces of the unit, 
7. The gauge unit is soldered to the cable described in reference ‘Ay 
the cable end being suitably prepared. The copper electrodes from the gauge 
are connected to the braid, and the silver centre electrode to the centre 
conductor, so that the gauge is now secured with its under edge approximately 
0.25" from the telcothene. The cable end and gauge unit is thoroughly washed 
in benzene or petrol, and the resistance of gauge and cable measured, 
8. The gauge is now dipped in a special rubberised wax (I.C,I. Hot Melt 
Adhesive L.D. 5257) heated to a temperature of 130°, the cable end being 
immersed to a depth of 0,1" approximately, The wax is allowed to cool, 
and dipping continued, until the gauge and cable end is embedded in a blob 
of wax. The wax is completely removed from the crystals and is tapered 
back eeece 
