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From the earliest days of telephony the evaluation of message circuit noise has been an important problem. Of particular interest has been the evaluation of noise originating from harmonic components present in power system voltages and currents. Magnetic fields set up by power currents, and electric fields set up by power voltages, tend to induce voltages in nearby telephone conductors. Even a small transfer of power at harmonic frequencies in the voice band can cause trouble. For example an input of 400 Â¿watts (micromicrowatts), or 26 db (decibels) above reference noise, into a telephone set at 1,000 cps (cycles per second) will give an audible tone. To put such a tone on all of the 60 million telephones in the United States at once would require just 24 milliwatts, transmission losses neglected.