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32.

[[underline]] DIRECT-LIFT IMPULSE-METER [[/underline]].

A section of the direct-lift impulse-meter is shown in Fig. 20(b). It is also shown in the photograph Fig. 14, at [[underline]] A [[/underline]]. A small cylinder [[underline]] A [[/underline]] of aluminum of 1.46 grams mass, hollowed at one end for lightness, was turned down to slide easily in a glass tube [[underline]] G [[/underline]]. This tube, [[underline]] G [[/underline]], was fastened by Khotinsky cement to an iron wire [[underline]] W [[/underline]], which was in turn fastened to the yoke [[underline]] Y [[/underline]], Fig. 21, so that the glass tube, [[underline]] G [[/underline]], was held in a vertical position, between the chamber and the wall of the 3-inch pipe -- similarly to the tissue paper. Two small wires [[underline]] C [[/underline]], [[underline]] C [[/underline]], of spring brass were cemented to the top of the aluminum cylinder, the free ends just touching on opposite sides of the glass tube. The inside of the glass tube was smoked with camphor smoke above the point marked [[underline]] X [[/underline]], so that a record was made of any upward displacement of the aluminum cylinder. The cylinder was prevented from dropping out of the glass tube by a fine steel wire, [[underline]] w [[/underline]], cemented to the tube and extending across the lower end.

The theory of the direct lift impulse-meter is given in Appendix C, p.75. From the theory, we may derive an expression for the ratio, [[underline]] Q [[/underline]], of the momentum given the gun by the gaseous rebound, to the observed momentum of the suspended system.

There are two disadvantages of this form of impulse-meter. First, friction acts unavoidably to reduce the displacement. Secondly, any jar to which the apparatus is subjected on firing will cause the aluminum cylinder to jump, and thus give a spurious displacement. This latter fact rendered the meter useless for experiments in which direct lift of the chamber took place, as there was always much jar when the heavy chamber fell back, after being displaced upward.

This impulse-meter, it will be observed, gave a mean measurement