726 
where KA [aucoulomb/(1b/in$)] is the constant for the gauge uscd, hy and hy 
the, pressure height and step height, respectively, and the other quantities 
are as defined previously. When the voltage step is used in place of the 
Q-step cclibration, Cy becomes C, the total capacity in the gauge circuit. 
The determination of the impulse and energy factor involves, the evaluation 
of fr dt and {Pe dt, usually by means ot the trapezoidal rule. This re- 
quires recording the valucs of the pressure at a large number of times from 
the pressure-time curve. The following methods have oecn found most satis- 
factory for this work on a routine basis, 
(i) Micrometer microscope. A micrometer microscope with 2 calibrated 
stage for moving the film at right angles to the micronetcr in the eyepicce 
was formerly used for analysis of all records, The film was aligned on the 
stage so that the pressure axis was measured by the micrometcr cyepicce and 
the time axis was measured by the scale on the stage. The ocular units of 
the eyepiece were transposed to pressure units hy comparison with the height 
of the step, while the time units of the stage scale were transposed into 
time units by comparison with the standard timing wave on cach filn. 
These records wore usually integrated graphically in terms of the 
ocular and stage units and transformed into absolute units after intcgra~ 
tion. When desired the set of points was plotted on graph paper in terms 
of absolute units. 
(iL) "Harp", A device was developed for obtaining a photographic 
projection of the pressure-—time curve superimposed on a two-dimensiondl 
grid adjusted to read absolute pressure and time units, This device, which 
is knowm as the "Harp" (Fig. 18), consists of a photographic enYarger with 
one fixed and one adjustable grid at right angles, immediatcly over an 
easel holding a shect of photographic enlarging paper. The film is placed 
in the enlarger with the pressure axis parallel to the fixed grid lincs, 
and the enlargement factor is adjusted until the fixcd grid reads convenient 
units of the time scale. This involves comparison with the time base which 
was placed on the record at the time the shot was made, 
After the time scale is adjustcd, the image of the Q-step is thrown 
on the paper and the adjustable grid is moved so that it will read in con- 
venient pressure units. This is done by calculating the pressure that 
corresponds to the step height, using the formula: 
GENE 
Pe A ° 
In cases where tho voltage stcp,is. used, Cy becomes the capacitance C of the 
gauge=cirduit cable. The adjustmont of the horizontal grid is made so that 
the step reads that particular pressure in absolute units on the grid. 
