RECORDING TR^iNSMISSION MEASURING SYSTEM 



27 



near the large one. This magnet induces eddy currents in a vane 

 which is attached to an extension of the pointer, thereby acting as a 

 brake to control the damping of the moving system. The ordinary 

 meter with a uniform airgap does not need an auxiliary damping at- 

 tachment as suitable damping can be obtained by eddy currents in- 

 duced in the moving coil as it turns in the airgap. The non-uniform 

 airgap gives a variable damping and the external damping device is 

 provided to equalize this variation. 



The heat-sensitive paper is also sensitive to friction and can be 

 marked by pressure with a small wire, a characteristic which is utilized 



Fig. 4 — Recorder mechanism showing moving system. 



to make each recorder rule its own db scale as a record is made. Cheap 

 plain unruled paper is used and a high accuracy of calibration is ob- 

 tained by making the ruling devices adjustable. Fig. 5 shows the 

 ruling devices which consist of loops of spring wire. While the paper 

 used in the recorder is sensitive to both heat and friction it will stand 

 handling without injury. 



The paper is 6 inches in width. Two rates of paper movement 

 are used in ordinary testing — 10 inches per minute for transmission 

 frequency measurements where speed is important and 6 inches per 

 hour for long-period observations. The paper moving mechanism is 



