PHYSICS: WEBSTER AND THOMPSON 
261 
comparing the properties of various powders (which we had occasion to do^ 
see figure 3) the rapidity, duration of burning, maximum pressure, main- 
tenance of pressure, total impulse, and the point and pressure at which the 
bullet leaves the barrel, and the time in the barrel can all be shown with the 
p, t curve. By mechanical integration we find the curve of velocity and 
distance and by combining the distance with the pressure we obtain the p, s 
or indicator curve, shown in figure 4. A method is now under experiment to 
obtain the p, s curve directly whether on a small or large gun. 
The advantages of the gauge used for maximum pressure measurement are 
also evident: 
1. After the calibration of the spring only one measurement is necessary; 
namely, the length of the straight line on the film or paper which measures 
the pressure. 
2. The length of this line is usually 50 mm. or more, permitting quick and 
accurate measurement. 
FIG. 2 
3. The same instrument is used for every measurement; that is, the flexure 
of the spring is only temporary. The spring returns absolutely to the zero 
position there being no permanent set or elastic hysteresis. 
4. The operation can be carried out quickly and inexpensively. 
5. Permanent record is provided, reading pressures directly on the film or 
print, and if desired the calibration can be photographed directly on the film. 
In order to show experimentally that the static calibration would give 
accurate results which could not be used in measuring impulsive forces, a 
calibration was carried out by the use of shock or impulsive forces. The /, t 
curve was obtained on a rotating film as in the ordinary indicator experiment, 
when a known weight dropped from a known height struck a piston bearing 
on the spring of the gauge. 
In figure 3 are shown a number of curves indicating the properties of dif- 
ferent powders furnished by the Winchester Repeating Arms and the DuPont 
Companies, together with a sample of German anti-tank gun powder fur- 
nished by Major Anthony Fiala of the Springfield Armory. 
