304 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1910. 



parts by passing it through two equal lamp resistances, each of which, 

 at the Pittsburg testing station, consists of a series of five 16-candle- 

 power lamps. These leads are then, after independently traversing 

 the cartridge file at the initial and final points, jointly connected to 

 one of the poles of the primary coil of the induction coil through 

 which the current passes to the return conductor. The secondary 

 coil of the induction coil is then connected by one pole to the two plat- 

 inum terminals and by the other pole to the base supporting the drum 

 as described. As is well known, in the induction coil any change of 

 tension in the primary coil sets up an induced current in the sec- 

 ondary coil, and this mutual induction between the coils results in 

 the production of a higher potential difference at the terminals of 

 the secondary one so that sparks of considerable length and intensity 

 may be obtained. 



The vibration tachometer, by which the speed of rotation of the 

 drum is measured, is connected to an auxiliary shaft which engages 

 the main shaft of the drum by gears, thus preventing any irregularity 

 in recording the speed due to slipping. This tachometer measures 

 the number of rotations of the drum, and as the circumference of the 

 drum is accurately known, the distance which any point on the pe- 

 ripherjT- travels may easily be calculated. Hence, at the highest speed 

 of 105 revolutions per second, the distance of travel is 52.5 meters. 

 At 50 revolutions it is 25 meters. At 86 revolutions it is 43 meters 

 per second. With this number of revolutions it is possible with this 

 instrument to measure the one-four-million-three-hundred-thousandth 

 part of a second of time. 



A more recent and simpler method of measuring the rate of det- 

 onation is that devised by M. d'Autriche,^ which was described at the 

 congress in London, in 1909, by Dr. A. M. Comey, as follows : 



The method of M. d'Autriche depends upon the use of a special detonating fuse 

 having a uniform velocity of 6,000 meters per second. A suitable length of fuse, 

 according to the length of the column of explosive to be tested is taken for the 

 test and the exact middle of the fuse is determined by measurement and marked. 

 A fulminate cap containing a charge of 15 grains (1 gram) is slipped over each 

 end of the fuse and crimped securely. The fuse is then laid upon a piece of 

 32-pound sheet lead (1* inches by 15 inches by * inch) (38 by 380 by 13 

 millimeters), so that the center of the fuse is about in the center of the sheet of 

 lead, and the point coinciding with the middle point of the fuse is marked 

 plainly on the sheet lead (M). The fuse passes along the entire length of the 

 sheet of lead, and its ends are bent around so that they nearly meet. The two 

 ends of the fuse covered with the detonating caps are inserted a short distance, 

 at two points, into the column of explosive, the velocity of which is to be tested, 

 and the distance between these points accurately measured. This may be called 

 (A). A fulminate cap with fuse or electric connections is placed in one end 

 of the stick of explosive. When this cap is detonated, the explosive wave 

 proceeds through the detonating fuse in both directions and meets at a point 



' Comptes rend. 1!,3, 641 and Hh, 1030. 



