542 BELL SYSTEM TECHNICAL JOURNAL 



tainable from a distributor driven by the 25-cycle tuning fork used in 

 the measuring set described above. The frequency used here for 

 modulation is exactly 1/20 that used in the other set. 



Modulation is accomplished mechanically by means of a commutator 

 and resistance potentiometer arranged as shown at the left end of 

 the figure. The commutator brushes are rotated at a speed of \}^ 

 revolutions per second. The carrier frequency is connected to the 

 potentiometer as shown so that the brushes as they pass over the 

 commutator segments will pick ofif various voltages from the poten- 

 tiometer, and on the completion of one revolution the resultant current 

 at the output is equivalent to a cycle of complete modulation of the 

 carrier such that both sidebands are transmitted with the suppression 

 of the carrier frequency. The potentiometer has been designed with 

 steps in such a way that, for all practical purposes, a modulation of 

 pure \]4: cycles is obtained, the higher harmonics in the modulated 

 wave being so far removed from the fundamental and relatively so 

 small that they are negligible. 



As only the two sidebands of the modulated wave are transmitted 

 here, the current which results from detection of this transmitted wave 

 at the receiving end will have a frequency of 2}^ cycles or twice the 

 modulating frequency. Another set of commutator brushes is re- 

 volved over a set of segments, somewhat similar to that already 

 mentioned, with a speed exactly twice that of the first, namely 2}i 

 revolutions per second. The output of the detector is connected to 

 these brushes and transmitted through the potentiometer shown con- 

 nected to the segments of the commutator to a very sensitive gal- 

 vanometer. The result of this arrangement is effectively a 2^-cycle 

 modulation of the 2^-cycle current received from the detector and as 

 a result of this modulation the current received by the meter will 

 consist of a d.-c. component and a 5-cycle component, the relative 

 amounts of each depending on the relation of the commutation to 

 the phase of the received current. The brushes of the first commutator 

 may be rotated at any instant relative to those of the second by a 

 manual adjustment and their position relative to some arbitrary point 

 noted. The galvanometer is arranged so that it does not respond 

 readily to any except direct current. There is a particular position of 

 the commutator brushes (and another 180 degrees removed from it) 

 which will give no deflection in the galvanometer. 



The particular details of measurement are considerably different 

 from those of the preceding circuit, but in principle the arrangement 

 is much the same. With a resistance line between the sending and 

 receiving terminals the adjustable brushes are shifted until no deflec- 



