Accurate Dispersions for Thirteen Pulsars

We determined the dispersions of thirteen pulsars with fractional accuracies between 2 X 1O~ and 1 X 1O~ from observations of strong individual pulses between 112 and 170 MHz. No variation with time or with pulse strength was detected. For six pulsars the dispersion obtained between 170 and 142 MHz differed from that obtained between 142 and 112 MHz by less than 1.0 X 1O'~ Hz, a value that sets an upper limit of about 1O~ cm6 pc to the emission measure of dense plasma along the paths to these pulsars. I. OBSERVATION