306 BELL SYSTEM TECHNICAL JOURNAL 



approximate orientation of the lines of electric force in the wave front 

 as well as the general directive pattern of the radiation. 



Figure 15 shows details of a crystal detector mounting suitable for 

 an indicator on a wave-meter. When silicon crystals are used, units 

 may be had that will hold their calibrations moderately well over 

 considerable periods of time. Thermocouples of both the cross wire 

 and deposited type have been used with moderate success. Also 

 diode and triode rectifiers have been tried. However, for general 

 laboratory use where simplicity and convenience are important the 

 crystal detector is perhaps best. 



Wave- Meters 



It is, of course, desirable in this work to know the frequency or 

 wave-length being used. The simple resonant chamber already de- 

 scribed enables wave-length to be measured accurately. However, 

 such a device does not give directly the wave-length in free space 

 since in these chambers phase velocities are in general greater than 

 ordinary light. It is true, of course, that a suitable conversion curve 

 can be prepared. However, it is often more convenient to use for a 

 wave-meter some form of a coaxial conductor system on which standing 

 waves may be measured. These will be very nearly at least the 

 length of the corresponding waves in free space. 



Figure 16 shows a wave-meter based on this principle. The con- 

 ductor (a) and the hollow cylinder(&) constitute the coaxial conductors. 

 A bridge (c) in the form of a conducting disc is made movable by means 

 of the threaded tube {d) w^hich passes over the central conductor (a). 

 This tube carries a millimeter thread engaged by the knurled head {e). 

 One complete turn of this head therefore raises or lowers the bridge 

 by one millimeter. If coarse adjustments are desired the head may be 

 disengaged from the threaded tube by a cam operated by the knob (J), 

 and the tube raised or lowered by taking hold of its extended portion. 

 The outer conductor or shell carries an open slot (g) through which 

 an index {h) attached to the shorting bridge (c) extends. This index 

 passes over a centimeter scale {i). The outer conductor is mounted 

 on a short piece of wave guide so that the apparatus may be clamped 

 in line with other apparatus. The inner conductor (a) extends through 

 a small opening in the section of wave guide far enough to extract from 

 the passing waves enough power for activating the wave-meter. This 

 coupling may be varied as needed by extending or retracting a third 

 small rod (j/) running through the center of central conductor (a). 



Resonance is indicated by a crystal detector {k) and d-c. meter. 

 This detector is only loosely coupled to the coaxial system by a small 



