440 BELL SYSTEM TECHNICAL JOURNAL 



Steel surface, such as results from an alternate oxidation and reduction 

 and by the addition of small amounts of aluminum to the zinc bath. 



Electroplated zinc deposits have the advantage of being applicable in 

 greater thicknesses than hot-dipped coatings. Electroplating methods 

 have made considerable progress, particularly in the wire field, with the 

 speeding up of plating rates as much as twenty-five fold. 



Bright zinc coatings have been developed in response to the demand 

 for improved appearance and this finish is gradually replacing the older 

 dull type. 



The protective value of zinc depends directly upon the thickness of the 

 coating. Experiments have listed environments in the order of increasing 

 attack as follows: rural, tropic marine, temperate marine, suburban, urban 

 and highly industrial. The resistance of zinc coatings to corrosion under 

 water depends largely upon the degree of circulation of the water and its 

 oxygen content. When a submerged zinc-coated armored cable is lapped 

 with jute, thereby stagnating the water, the capacity of zinc to resist cor- 

 rosion is increased. 



Cadmium plate has good color and is very satisfactory for indoor use. 

 It does not possess corrosion resistance equal to zinc under conditions of 

 outdoor exposure. Bright nickel coatings or semi-bright coatings requiring 

 mild buffing have largely replaced the older type of nickel coatings. 



A very promising process of protecting steel, known as "Corronizing" 

 consists in the application of a layer of nickel plate followed by either 

 zinc or tin. The duplex coating is heated to 700-1000°F. yielding alloys 

 practically free from pores which show high resistance to the salt spray 

 test. 



Seventy per cent of the production of tin plate is used in cans. The hot- 

 dipped process is old and well established but is being challenged by con- 

 tinuous rolling processes involving electroplating methods of application. 



Recent progress in the protection of metals by coatings of other metals 

 is largely in the direction of electroplating and continuous processes. 



Measurements of the Delay and Direction of Arrival oj Echoes from N ear- 

 By Short-Wave Transmitters.^ C. F. Edwards and Karl G. Jansky. Ob- 

 servations on pulses radiated by a high-power beam transmitter operating 

 in the short-wave range show that when the receiver is located within the 

 skip zone, echoes are observed having delays of from 1 to 50 milliseconds. 

 These echoes are the result of scattering and three different types may be 

 recognized, each arising from a different source. 



Echoes of the multiple type were found to occur the most frequently 

 and to have many of the characteristics of signals transmitted over long 



3 Pfoc. /.i?.£., June 1941. 



