120 BELL SYSTEM TECHNICAL JOURNAL 



including 60-cycle power panel, rectifier panel, meter and control panel and 

 centimeter wave oscillator panel with coaxial conductor output jack. All 

 high-frequency connectors are coaxial conductor units with plug tips. 



The measuring unit is shown in the two photographs; Fig. 1, assembled 

 and Fig. 2, disassembled. Two combination input-output heads are shown 

 in Fig. 2. These heads and tubing together with center conductor and plun- 

 ger are of coin silver. While the highest possible conductivity metal is 

 desirable, pure silver is mechanically too poor for spring fingers and bearing 

 surfaces and the alloy must be used. The good sliding contact properties 

 of silver are preserved but the conductivity is no better than that of copper. 

 Both heads are drilled, for input and output connections, flush with the 

 bottom of the cylindrical cavity terminating the tubing. 



Head ^1, shown attached in Fig. 1 and detached in lower right-hand 

 corner of Fig. 2, has a silicon crystal, mounted and insulated in a small 

 cylindrical holder which carries a tiny pickup loop, one side of which is 

 grounded to the cylinder. The total length of pickup conductor including 

 loop and crystal "whisker" is about one centimeter and no tuning is neces- 

 sary. The loop pickup can be adjusted by moving the holder in or out. 

 The d-c circuit is from an insulated pin on the holder through crystal to 

 apparatus body. 



The current input connection is through a coaxial plug which is tapped 

 across a fraction of a tunable half-wave line. This fraction consists of a 

 Y coaxial conductor terminated in a tiny feed loop; the remainder of the line 

 is an ordinary \" coaxial with sliding plunger. The line is used, well off 

 tune, as an input current amplitude control. The coupling with the cavity 

 in head is adjusted by moving the feed loop in or out. 



By inverting another half-wave coaxial with feed loop, so as to put the 

 crystal where the feed jack was, it is possible to use an externally mounted 

 crystal as in head ^ 2. For this head the input current ampUtude control 

 is obtained by using, as a feeder, a short \" coaxial tipped with a tiny loop 

 and a coaxial jack, at opposite ends. This coaxial is mounted in a spring 

 clamped bearing so as to permit a rotation of the plane of the loop. All 

 coaxials, except the measuring unit itself, are 72-ohm ones. 



There is no essential difference in operation between these two heads; 

 they are interchangeable. However, head ^ 1 is more convenient in ma- 

 nipulation, during the disassembly required to insert the dielectric sample. 

 (This sample is always positioned in the piece of tubing connecting to the 

 head.) 



An ordinary model 301 microammeter, low resistance, served as indicat- 

 ing instrument. By replacing the cr\'stal holder of head ;^ 2 with a loop 

 tipped coaxial and plug, a conventional double-detection radio receiver with 



