The R.M.S. value of the pressure was taken from the oscillogram by taking the square root of the average value of the squares of a number of equi-distant ordinates; the R.M.S. value of the current was obtained in the same way from the current oscillogram, and the two values multiplied together always gave a value agreeing to within 1 per cent. of that obtained by first multiplying the ordinates taken from the current and voltage oscillograms and then taking the mean.