216 PHOTOGRAPHY OF UNDERWATER EXPLOSIONS 



been reported by Campbell (16, 17), in which microflash units have 

 been employed. 



Another type of source employs discharge of a condenser through a 

 spark gap in the water. A spark arranged to act as a very small source 

 of high intensity and duration of a few microseconds has been developed 

 by Libessart (69). Magnesium electrodes are employed, the light be- 

 ing emitted from a tiny hole in one electrode, and the main discharge 

 circuit from a high voltage condenser is closed by auxiliary gaps excited 

 from a spark coil. This type of spark is particularly simple to use in 

 many applications, because it acts virtually as a point source, and sharp 

 records can be made on photosensitive material without use of a lens. 



As compared with explosive flash charges, electrical flash methods 

 are much more convenient for tests which can be performed on a lab- 

 oratory scale. This convenience is largely lost in field work with larger 

 charges, because of the limited light output without multiple units 

 and the increased electrical difficulties. For such work, the explosive 

 flashes are more useful, particularly if facilities for charge preparation 

 are available. 



The use of flash sources similar to those described has the disad- 

 vantage that a single unit gives only a single flash or requires a con- 

 siderable interval before it can be triggered again. This objection can 

 be met by the use of a source of high intensity and longer duration if 

 the film is moved rapidly past a slit and a "streak" photograph taken. 

 Experimental methods of this kind (72) have been extensively used in 

 studies of detonation waves, which are self luminous, and have been suc- 

 cessfully applied to measurements of shock wave velocities in water, 

 illumination being supplied by the self-luminous shock wave excited in 

 an argon-filled tube, as shown in Fig. 6.2. The advantage of the con- 

 tinuous time record from this method is of course obtained at the price 

 of limitation of the observable field by the slit. 



. D. Sources of greater duration. Flash sources or very high speeds of 

 mechanical motion are essential for stopping the motion of shock or 

 detonation waves which travel several feet in times of a millisecond or 

 less. For slower but still, by ordinary standards, rapidly changing 

 phenomena, such as the expansion of the gas sphere, a number of types 

 of light source are possible. Stroboscopic illumination with contimious 

 film motion and continuous high-intensity illumination using high-speed 

 motion picture cameras have both been used. The stroboscopic methods 

 are based on Edgerton's developments, and the lamps employed in the 

 General Radio power stroboscope, for example, have been used to give 

 flashes at rates up to 1,500 per second with exposure times of the order 

 of 30 microseconds. High-speed motion picture cameras employing 

 rotating prisms to provide the equivalent of intermittent film motion, 

 such as the Eastman High-Speed and Western Electric Fastax cameras, 



