730 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1956 



housing are of similar materials, and have almost identical temperature- 

 expansion coefficients, it is expected that pole-tip-to-drum clearance will 

 remain unchanged under normal conditions of service. The drum, which 

 is 12.8" in diameter, 10" long, and weighs 150 pounds, is dynamically 

 balanced and runs without sensible vibration. 



Commercial super-precision angular-contact ball bearings, two at 

 each end, are used to mount the drum in its housing. The lower bearings 

 are arranged to share the thrust load imposed by the weight of the drum, 

 and the upper bearings are mounted opposing each other, and are pre- 

 loaded one against the other. The upper bearings serve only as radial 

 constraints, the outer races being free to move axially. This type of con- 

 struction results in a finished unit having a total runout of only a few 

 ten-thousandths of an inch without the necessity of machining the drum 

 on its own bearings. For the experimental installation, the bearings were 

 grease-packed at assembly and can be expected to function satisfactorily 

 during any reasonable test period. If, however, such a drum unit were 

 made a permanent part of the telephone plant, other provisions have 

 been considered which wovdd insure adequate lubrication over a much 

 more extended period. 



The magnetic coating used on the drum is an electro-deposited alloy 

 of cobalt and nickel (90 per cent Co-10 per cent Ni) approximately 

 0.0003" thick. This coating was selected because of its hardness, strength, 

 uniformity, and desirable magnetic characteristics. The thickness of the 

 coating is such as to result in a satisfactory cell-size without undue sacri- 

 fice in output. The purpose of the brass sleeve mentioned previously is 

 to form a nonmagnetic surface between the magnetic coating and the 

 cast-iron core since, if the coating were applied directly to a ferro-mag- 

 netic material, its effectiveness would be greatly reduced by the shunting 

 effect of the base material. The brass sleeve also serves to facilitate plat- 

 ing the drum, since brass, unlike cast-iron, is amenable to the electro- 

 plating process. 



Read-Write Heads 



One of the read-write heads is shown in Fig. 9. The magnetic structured 

 consists of three rectangular bars of laminated material, arranged in theij 

 form of a triangle (as schematically represented in Fig. 2). Two legs of j 

 this triangle carry single-layer coils which are series-connected. These; 

 two legs also serve as pole-tips, being pointed at the end and separated ( 

 by an air gap. The third leg sorx-es to complete the magnetic circuit and, f 

 in assembly, is butted tightly against the other members by means of a-, 

 Icafspring. 



