26 BELL SYSTEM TECHNICAL JOURNAL 



the paper. The temperature of the wire is regulated to suit conditions ; 

 however, the maximum heat used is insufficient to char the paper even 

 when it is not in motion. This method of recording is particularly 

 satisfactory from a maintenance standpoint. A record is made almost 

 the instant the current is turned on and there is no danger of failure 

 of recording when the meter is not in continuous operation. 



The reliability is so great that it is not necessary for the attendant 

 making the test to see the recording meter while it is in use. Because 

 of this and the stability of the associated sending and receiving ap- 

 paratus, it should be possible to locate an instrument of this type at 

 some central point in an office and have it used by testers some distance 

 away. For such an installation it would of course, be necessary to 

 have auxiliary circuits for enabling any tester to determine if the system 

 is available for use, to indicate when a test has been completed and to 

 enable the recording meter and oscillator to be started from remote 

 points. Either the oscillator or the recording meter can be set in 

 motion by the operation of a key and, if desired, each device can be 

 made to stop automatically when the test has been completed. 



Circuit characteristics, such as transmission efficiency, speech vol- 

 ume, and noise are all measured in terms of the unit of transmission — 

 the decibel, referred to as the db— and the meters used in making 

 these measurements are calibrated in db. An ordinary voltmeter or 

 ammeter which has a uniform voltage or current scale will have a 

 logarithmic db scale since current changes corresponding with db 

 changes have a logarithmic relation. The logarithmic db scale is not 

 suitable for maintenance work as some of the divisions are unneces- 

 sarily large and others too small to be read accurately. The range of 

 the recording meter is about 26 db and the scale is divided into 2 db 

 divisions. Ten of these divisions have been made approximately 

 equal by a special design of the magnetic circuit of the instrument. 

 In the conventional moving coil instrument the moving coil rotates 

 in an airgap of uniform width and great effort is made to have the 

 flux distribution in this gap uniform. In the recording meter the 

 airgap is not constant but increases in width with the deflection of 

 the coil. With suitable shaping of the pole faces of the permanent 

 magnet and the iron core around which the moving coil turns, the 

 magnetic flux distribution in the gap causes the angular movement 

 of the coil to be approximately proportional to the current change 

 expressed in db. 



Figure 4 is a view of the moving system and the recording mechanism 

 swung out of the case and shows the moving coil and the large magnet 

 associated with it. It will be noted that a small magnet is mounted 



