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31.

[[underlined]] TISSUE PAPER DETECTOR [[/underlined]].

The detector for indicating the force of the rebound consisted of a strip of delicate tissue paper, [[underlined]] I [[/underlined]], Figs. 1[[strikethrough]]9[[/strikethrough]] ^[[4]] and 20(a), 0.02 mm. thick, with its ends glued to an iron wire, [[underlined]] W [[/underlined]], as shown in Fig. 20(a). This iron wire was fastened to the yoke [[underlined]] Y [[/underlined]], Fig. 15, and held the tissue paper, with its plane horizontal, between the chamber and the wall of the 3-inch pipe, [[underlined]] P [[/underlined]]. In many of the experiments, the paper was cut 1/3 the way across in two places before being used, as shown by the dotted lines [[underlined]] h [[/underlined]] in Fig. 20([[strikethrough]]b[[/strikethrough]] ^[[a]]). Since the tissue paper has very little mass, the tearing depends simply upon the magnitude of the force that is momentarily applied, and not upon the force times its duration; i.e. the impulse of the force. The tissue paper will tear, then, if the force produced by the first upward rush of gas, past the chamber into the space in the 3-inch pipe above the chamber, exceeds a certain value. This first upward rush of gas will, of course, produce a greater force than any subsequent rush, as the gas is continually losing velocity. Even though the magnitude of the force that will just tear the tissue paper be not known, it may safely be assumed that if the first upward rush does not tear the paper, the force due to rebound that acts upon the gun must be small compared with the impulse produced by the explosion of the powder.

It should be noted that the tissue paper tells nothing as to whether or not there are a number of successive reflections or rebounds gradually decreasing in magnitude; neither does it give information concerning the [[underlined]] downward [[/underlined]] pressure the gases exert upon the chamber tending to decrease the displacement, after they have accumulated in the space between the top of the chamber and the cap, [[underlined]] C [[/underlined]], Fig. 14.