MEASUREMENT OF PRESSURES 197 



Inasmuch as none of the standard types of coaxial cable has been 

 found satisfactory as regards cable signal, efforts have been made, both 

 in this country and in England to devise better special cables. One 

 such design, devised at the Taylor Model Basin (42), employes a ]/^ inch 

 outside diameter soft copper tubing with 1/16 inch bore. A central 

 copper wire (#22 or smaller) is used as central conductor, and the tube 

 is filled with ceresin wax for dielectric. This filling may be accomplished 

 in a variety of ways, one of the most effective being to heat the tube by 

 an electric current, pumping fluid wax into the cavity, and cooling grad- 

 ually to prevent formation of voids. Cables of this type have been used 

 with considerable success in measurement of shock wave and bubble 

 pulse pressures. No two cables are exactly alike, and tests of all sec- 

 tions of tubing used are necessary if a minimum of cable signal is to be 

 realized. Good cables of this construction develop one-fifth to one- 

 tenth the signal of the best of a number of rubber dielectric cables 

 tested at Woods Hole, and these cables have been found adequate to 

 reduce cable signal error on shock wave measurements to the order of 

 one or two per cent. Similar cables used with larger gauges in bubble 

 pulse measurements give rise to errors of the order of five per cent. It 

 might appear that use of these cables would be hopelessly difficult in 

 large charge work where the distance from gauge to recording equipment 

 is hundreds of feet, but only the fraction of this length corresponding to 

 the distance travelled by the pressure wave in the desired time interval 

 need be composed of low signal cable; for the rest, standard types of 

 cable may be used. A somewhat similar type of cable, devised by the 

 Stanolind Oil and Gas Co. Research Laboratory, consists of a con- 

 ventional coaxial cable except for an outside lead sheath replacing the 

 usual copper braid. This cable has been found to be considerably bet- 

 ter than the standard construction, but is usually somewhat inferior to 

 the copper tubing design. 



A British cable known as "Telconax," developed by the Telegraph 

 Construction and Maintenance Co. for air blast work, has been found 

 to have very low cable signal. This cable is much like a coaxial cable 

 except for the use of a relatively soft and porous dielectric. Because of 

 its inferior mechanical properties, use of this cable in marine work seems 

 doomed to failure except under ideal conditions. 



The probable origin of cable signal at the interface between the 

 dielectric and outer shield of conventional cables suggests the possi- 

 bility of reducing the signal by making the dielectric surface conduct- 

 ing, either by use of colloidal graphite or by use of conducting rubber 

 near the outside surface. Several types of cable along these lines have 

 been or are being developed. At the time of writing, preliminary un- 

 derwater tests have been made indicating that such cables are equal or 

 superior to copper tubing cable but further tests are needed. 



