The measure is twice the RMS value.

  See:
  Gary Franklin and Toy Hatley, "Don't Eyeball Noise...Unique Tangential
Method," article in Electronic Design 24, November 22, 1973, pgs.
184-187
  Siocos, and Quinn, "Oscilloscope Method for Measuring Signal-to-Noise
Ratios," J. SMPTE, Feb. 1967, p. 121
  Michael E. Gruchalla, "Measure Wide-Band White Noise Using a Standard
Oscilloscope" in EDN in 1980:     Gruchalla, "A Simple and Effective
Procedure for Measurement of Wide-Bandwidth Noise," in Interference
Technology Engineers' Master, item 1989, pgs. 272-288. 

 I have copies of the first and last items and used them to quote
references for the other two.  The articles describe the method and the
theory behind it.

Charles R. Patton 
        >>>>>>>>>>>>>>>>>>>>>>>><<<<<<<<<<<<<<<<<<<<<<<<<<

Karl Cunningham wrote:

> Now adjust the vertical position of one of the channels until the traces
> overlap in such a way that the brightness of the display between the two
> channels is almost constant. ... The number of divisions between the traces times the vertical
> sensitivity is a measure of the noise of the signal.
> 
> Unfortunately, I don't remember if this number is supposed to be peak, RMS,
> or what.
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