532 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1952 



materials. The followdng premises guided the development program: 



(1) The recording medium should be subjected to the least possible 

 physical manipulation in use to minimize failures. To accomplish this it 

 was decided to develop the recording medium in a form suitable for use 

 on the surface of a rotating cylinder and to use a helical recording track 

 on this surface when the message length required more than one revolu- 

 tion of the cylinder. It was hoped that with this arrangement, physical 

 failure of the recording mediimi would be eliminated, and the service 

 life would be determined by the wear occurring between the medium 

 and the magnetic pole pieces. 



(2) The recorcUng medium should exhibit some compliance to facilitate 

 intimate contact with the magnetic pole-pieces. 



(3) The transmission quality should meet present-day telephone stand- 

 ards for transmission of speech. The liigher quality necessary for the 

 recording of music, while desirable, should not be considered a re- 

 quirement. 



A number of experimental powdered media Avere prepared and tested. 

 These all utilized commercially available iron oxide powder with a 

 coercive force of approximately 250 oersteds, and the samples included 

 coated media, made by dipping, spraying and doctoring the coatmg on 

 various base materials, and impregnated media, prepared by mixing 

 the oxide in the base material and forming the mixture. 



A medium consistmg primarily of an elastic rubber band impregnated 

 with magnetic particles was found to be particularly suited to appli- 

 cations requiring long life in continuous service. A study of compounding 

 and manufacturing processes for this medium was made by the rubber 

 products group at these Laboratories under the direction of H. Peters, 

 and the compound which evolved consists primaril}^ of sjmthetic rubber 

 loaded with magnetic iron oxide, and containing small amounts of lubri- 

 cants, inliibitors and curing agents. The compound is decidedly rubber- 

 like in character, and is utilized in the form of seamless bands about 

 ^ to I inches thick, w^hich are stretched over the surface of cylinders 

 about 10 per cent larger than the bands. 



The bands are prepared by thoroughly milling together the following : 



100 Parts by weight type GN neoprene 

 100 Parts by weight magnetic iron oxide 



5 Parts by weight zinc oxide 



4 Parts by weight magnesium oxide 



2 Parts by weight paraffin 



