METHODS OF TESTING EXPLOSIVES MUNROE. 303 



wires are inserted through perforations in the tube and the cartridge 

 file at a distance of 1 meter apart, and these wires are separately led 

 to a chronogTaph. An electric detonator of the usual type and grade 

 is inserted in one of the end cartridges of the file ; the tube, as now 

 arranged, is suspended in the firing chamber ; the copper wires which 

 pass through the explosive are connected up to the chronograph; and 

 the charge is fired. 



The chronograph which records the time that elapses between the 

 rupturing of the wire nearest the detonator and the wire 1 meter dis- 

 tant from it is known as a Mettegang recorder, and is shown in plate 

 10. The primary components of the Mettegang recorder are a soot- 

 covered bronze drum so connected to an electric motor that it may be 

 caused to revolve at any desired speed up to 105 revolutions per sec- 

 ond ; a 200-volt D. C. electric motor provided with a rheostat for con- 

 trolling its speed ; a vibration tachometer so connected to the bronze 

 drum that the number of revolutions of the latter in unit of time are 

 accurately measured for any speed between 50 and' 105 revolutions 

 per second; induction coils which may receive their electric current 

 from electric-lighting circuits having terminal pressures of from 

 about 110 to 220 volts; and platinum terminals placed about one- 

 fourth of a millimeter from the surface of the rotating drum, and in 

 circuit with the induction coils, by which electric sparks are so pro- 

 jected against the surface of the drum as to disturb its sooty cover- 

 ing and produce a tiny bright spot at the point of impact, which 

 spot may be easily perceived by the aid of a microscope attached to 

 the drum. 



The drum is 500 millimeters in circumference. The edge of this 

 drum is provided with 500 teeth which may be made to engage an 

 endless screw. A pointer attached to this screw passes over a dial 

 reading to hundredths, and it thus enables one to read the distance 

 intervening between the spots produced on the soot-covered surface 

 of the drum with great precision. The drum is provided with six 

 platinum terminals which are held by an insulated arm that may be 

 so moved as to bring the points within any desired distance from the 

 drum, and each one of these points may be put in series with one of the 

 induction coils while the other end of the electric lead is grounded to 

 the drum through the base which supports it. Only two of these 

 platinum terminals are used in any single-firing trial for the deter- 

 mination of the rate of detonation in a given explosive, while the 

 other four are held in reserve for future use. 



To operate this method of ascertaining the rate at which det- 

 onation when once initiated is transmitted through a column, or file, 

 of an explosive, the electric current which is used as the medium for 

 transmitting the record is, as taken from its source, divided into two 



