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BELL SYSTEM TECHNICAL JOURNAL 



cross-talk through the shunt or directly to the crosstiilk from the 

 disturbed line. Errors which might be introduced should line noise be 

 present for only one condition of the test are in this way eliminated. 

 Measurements are made by first impressing the alternating current 

 tone on liie disturbing circuit and then on the meter and adjusting 

 the shunt until the annoying efifect of the tone heard in the disturbed 

 circuit is judged to be the same as that heard on the meter. The 

 crosstalk meter is calibrated in crosstalk units, one unit being defined 

 as the ratio of one millionth between the current at the terminal of 

 the disturbed circuit and the current at the terminal of the disturbing 

 circuit, pro\iding these currents are transmitted into like impedances 

 and distortion of tlie speech sounds is not involved. 



Fig. 21 shows the sini|)lilied cinuit of a noise measuring set arranged 

 to measure metallic noise on a telephone circuit. As in the case of 

 the crosstalk set, a shielded transformer is used to connect the set 

 to the circuit under lest which can be adjusted to give the proper 

 impedance relations. Willi liu' swilcji thrown tow.uds "liiu'" ilu' 

 receiver is connected lo the ciriiiil undtT test ant! anN' noise on this 

 circuit ol)ser\i(i. When the >\\ili]i is thrown towards "shunt" an 

 artificial noi.se ciurenl prudiuti! by .i xibrator is impressed on the 

 receiver through a shunt. H> aiiernalei>- throwing the switch from 

 the line under test to the sluml circuit, the shunt is adjustetl until the 

 interfering effect of the noise on the line and from the shunt are 

 judged to be equal. The reading of the shunt which is calibrated in 

 noise units gi\'es a measure of the amount of noise in the circuit under 

 test. In the commercial form of instrument used in the plant, the 

 circuit is arranged so that both metallic noise and noise to ground 

 can be readily measured. Where noise is present on circuits, instru- 

 ments are also available for anaKving ihe wave shapt', thai is, de- 



