194 BELL SYSTEM TECHNICAL JOURNAL 



the furnace tube and increases the efficiency of the pumping system 

 by removing water vapor and carbon dioxide from the gases to be 

 pumped out. Rough measurements of gas pressure are made with a 

 McLeod gauge on the pump side of the hquid air trap, but measure- 

 ments of low pressures are made with an ionization manometer on 

 the furnace side. 



With this furnace, and using clean alundum parts, metals can be 

 melted with very little contamination of any kind. Copper and iron 

 have been melted under a pressure never rising above 1 X 10"^ mm. 

 Hg and having a final value of 1 X 10~^ to 1 X 10"'' mm. Hg. Even 

 lower pressures can no doubt be obtained by cooling the pyrex furnace 

 tube more effectively, as by use of a water jacket, and by using gas 

 absorbing chemicals such as activated charcoal. 



It has been found with this furnace that, even when melting iron, 

 the temperature of the glass furnace tube never rises above 150° C. 

 Since pyrex glass does not soften below 500° C, it should be possible 

 to melt metals with higher melting points than that of iron. 



In this furnace, a horizontal boat rather than a crucible is used to 

 hold the sample for two reasons: First, the surface area of metal is 

 greater and the pressure head of metal less so that the melt is degasified 

 more rapidly than it would be in a vertical crucible. Second, very 

 much smaller and less troublesome pipes form in horizontal ingots 

 than in vertical ones. ^ 



Commercial Vactmm Melting 



Vacuum melting on a commercial scale was developed in Germany 

 during the war because of the need for a method by which the compo- 

 sition of alloys could be accurately controlled. The scarcity of 

 platinum, for instance, necessitated the commercial production of 

 substitute alloys for thermocouples which could be used, without 

 calibration, in direct reading instruments. From the small furnaces 

 used for this work, vacuum furnaces capable of handling up to four 

 tons have been developed. 



A brief description of one of these furnaces and of the obstacles 

 encountered in their development has been given by Rohn in his 

 publications.-^ The furnace described by Rohn operates by high 

 frequency induction. It has a horizontal, ring-shaped melting cham- 

 ber surrounded by the primary induction coils and an iron core. The 

 whole is enclosed in an air-tight casing arranged so that it can be 

 tilted about a horizontal axis. Two molds are fastened on opposite 

 sides of the casing through air-tight connectors. When the charge is 



^-i Rohn, Zrit.fiir McUiUknmle, 21, 12 (1929). Enoinecring, Oct. 18, p. 512 (1929). 



