356 BELL SYSTEM TECHXICAL JOLRXAI. 



high permeability at low magnetizinR forces. The relative propor- 

 tion of nickel and iron in permalloy may be varied through a wide 

 range of additional elements as, for example, chromium may be added 

 to secure high resistivity or other desirable properties. On account 



Fi};. 1 — rtriiialloN -Loaded C'alile. .Al)o\c, section of deep sea ty[>e showing con- 

 struction. Below, section of core showing permalloy tape partly unwound. 



of its extremely high initial permeabilit\- a ihiii la\er of |)ermalloy 



wrapped around the copper conductor of a cable great 1\- increases its 



inductance even for the smallest currents. 



In the case of the New York-Azores cable the permalloy tape is 



composed of approximately 78\2% nickel and 21 J o*^ iron and gives 



the cable an inductance of about 54 millihenries per nautical mile. 



An approximate \alue of the initial permeal)ilit\' of the permallo\' 



in that cable ma\- be got by assuming the helical tape replaced b>' a 



continuous c\'linder of magnetic material of the same thickness.' 



This material would have to ha\e a jxTmeabiiity of about 2,300 to 



give the observed inductance. A belter appreciation of the extraor- 



dinar\- properties of the new loading material may be obtained b>' 



(■om|)aring this [lermeabiiity with tlial w liich has pre\'iously been 



obtained with iron as the loading material. The Key West-Ha\ana 



telephone cables are loaded with 0.1)0^ inch diameter s<jft iron wire. 



The permeability' of this wire, which was tlie ln'sl which coiilii i)e 



obtaini'd coinmercialK- wlu-n tli.il lalilr was maili', i> onK alxml II."), 



'The true initial pernieaMlity is slightly higher. To loiiipiite it, account nuist 

 he taken of the fact that, contrary to what has lH.'en sometimes assumed, the mag- 

 netic lines of in(hiction in the tape do not form doseil loops around the wire but tenil 

 to follow the tape in a helical path. The pitch of the helical path of the lines of 

 induction is slightly less than that of the permalloy tape with the result that a line 

 of induction takes a numher of turns around the conductor, then crosses an airgap 

 Iwlween two adjacent turns of tape and continues along the tape to a point where 

 it again slips hack across an airgap. O. K. Buckley, British Patent No. 206,104, 

 March 27, 1924, also K. U. Wagner, K.N.T., Vol. f, No. S, p. 1.S7, 1924. 



