654 BELL SYSTEM TECHNICAL JOURNAL 



Comparison of Natural and Synthetic Hard Rubbers.^ G. G. Winspear, 

 D. B. Herrmann, F. S. Malm, and A. R. Kemp. GR-S, nitrile, and natural 

 hard rubbers are compared as regards compounding, processing, vulcaniza- 

 tion, and physical and dielectric properties. Natural rubber and GR-S 

 compounds intermediate in sulfur content between hard and soft rubber also 

 are compared. GR-S and nitrile rubber compositions suitable for commer- 

 cial ebonite fabrication are described. Extensive breakdown of the basic 

 copolymers has little effect on the physical properties of synthetic ebonites. 

 The time required for the beginning of exothermic reaction in vulcanization 

 is longer for GR-S than for natural rubber ebonites. Rockwell hardness is 

 greater for GR-S. Some GR-S ebonites are penetrated to the same depth 

 as natural ebonites, with a greater tendency toward instantaneous recover^'. 

 The two are similar in impact strength, but the ability to withstand a sharp 

 bend is characteristic of natural ebonites alone. The latter are superior to 

 GR-S ebonites in heat deformation below 60° C., but above this temperature 

 the reverse is true and nitrile ebonites are superior to both. GR-S ebonites 

 are more stable and nitrile ebonites less stable chemically than natural 

 ebonites. GR-S ebonite dust as a filler increases brittleness. A diatoma- 

 ceous earth improves the processing properties of GR-S hard rubbers. The 

 adverse effect of ultraviolet light on surface resistivity is reduced when a 

 GR-S hard rubber is filled with whiting. Natural and GR-S hard rubbers 

 are alike in dielectric behavior. 



Signal and Noise Levels in Magnetic Tape Recording^ D . E. Wooldridge. 

 The primary object of the work described here was to determme what proper- 

 ties of the tape and associated magnetic elements are responsible for the 

 noise and signal output levels of magnetic recordings and, if possible, to dis- 

 play in specific equations the pertinent relationships connecting noise and 

 signal levels with the physical properties of the tape and polepieces. In the 

 course of the study, methods appeared for decreasing the noise and increas- 

 ing the useful signal reproduced from magnetic tape. These methods and 

 some of the use that Bell Telephone Laboratories and Western Electric have 

 made of them are mentioned in the discussion. While some of the work 

 described in this paper has implications for more than one t}'pe of magnetic 

 recording process, perpendicular recording on tape is the actual subject mat- 

 ter dealt with. In every case discussed, the record medium was 0.050 inch 

 wide and 0.0022 inch thick. Except where otherwise noted, a chrome-steel 

 tape was used at a speed of 16 inches per second. 



^ Indus, and Engg. Chemistry, July 1946. 

 ^ Elec. Engg., Trans. Sec, June 1946. 



