126 BELL SYSTEM TECHNICAL JOURNAL 



The first of these problems was solved by the development of a 

 new metal to glass seal. In making this seal the glass and metal parts 

 are brought into contact while hot, the temperature being high enough 

 for the glass to wet the metal. The part of the metal in contact with 

 the glass is made so thin that the stresses which are set up when the 

 seal cools are not great enough to fracture the glass or to break it 

 away from the metal at the surface of contact. Seals made in this 

 way are sufficiently rugged to stand repeated heating and cooling 

 from the temperature of liquid air to that of molten glass without 

 deterioration. 



A seal employing the same principle but different in form is also 

 used at the point where the leads carrying the filament current pass 

 through the glass walls of the tube. The lead is made of copper 0.064 

 in. in diameter and passes through the center of a copper disk, 0.010 

 in. thick, the joint between the lead and the disk being made vacuum- 

 tight by the use of a high melting solder. The disk is sealed to the end 

 of a glass tube which is in turn sealed into the glass wall of the vacuum 

 tube. 



In exhausting the tubes it has been found necessary to subject all 

 the metal parts to a preliminary heat treatment in a vacuum furnace 

 during which the great bulk of the occluded gasses is removed. By 

 this method the time of exhaust can be considerably reduced but the 

 vacuum conditions to be met are so stringent that the final processes 

 of evacuation must be carefully controlled and often occupy as much 

 as twelve hours. 



The tubes are operated at a plate voltage of 10,000 volts and are 

 capable of delivering 10 kw. at this voltage in a suitable oscillatory 

 circuit. For this performance an average electron current of 1.35 

 amperes is required. The total electron current that the filament 

 must be capable of supplying to insure steady operation is about 6 

 amperes. 



When the tubes are used to amplify modulated currents with large 

 peak values such as are characteristic of telephone signals it is essential 

 that the maximum electron current through the tube shall be several 

 times the normal operating current and therefore to insure the neces- 

 sary high quality of transmission these tubes are operated for tele- 

 phone purposes with an average output of about 5 kw. 



The Receiving System 



In the method of transmission ordinarily employed in radio telephony 

 by which the carrier and both side-bands are sent out from the trans- 

 mitting station and received at the distant end, detection is readily 



