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adapted, a tight Tufnol arm being attached to the moving armature. u BA brass studs on the moving 

 arm and on a fixed Tufnol arm serve as contacts. A gap of just over -ith inch in the open position has 

 been found sufficient when working at 8,000 voUs to prevent the train of sparl<s passing across the 

 points of the rotary gap. The total time of operation of this relay is about lo milliseconds, although 

 the contacts are only moving for about 3 milliseconds. 



Th e Can e ra. 



The underwater camera is shown diagramnatlcal ly in Figure u; Figure 5 is a photograph of the 

 camera removed from its watertight housing, it is, in effect, two separate drum cameras mounted side 

 by side. This enables a sufficient nur*er of pictures to be tal<en without having an abnornally large 

 drum. It also permits the two groups of flashes to be timed solely with regard to the phenomena to be 

 observed and without regard to the relative position of the two sets Lf pictures on the film. 



The two 7 inch diameter duralumin drums A carrying 35 m.m. film are mounted side by side on a 

 commcn spindle and driven at speeds up to 3,000 r.p.m. by a belt drive B from a 12 volt car dynamo C 

 used as a motor. The ball races D in which runs the drum spindle are housed in a pair of brackets 

 formed by angle iron members E welded rigidly to the bed plate F. This design combines lightness with 

 great rigidity. 



The two lenses G are Taylor H.bscn "Speed Panchrc" lenses with an aperture of f/, and a focal 

 length of. 58 m.ms. Between the lens and the film a single duralumin vane H, 6/1,000 inch thick, acts 

 as a shutter and is activated by a solenoid and lever system 1. The arrangement was the simplest which 

 could be put in the available space. On the application of about 18 volts to the solenoid the shutter 

 cpens in about 15 mill iseccnds; on removal cf the voltage the- shutter closes in a similar time. under 

 present conditions cf use this gives ample time for one shutter tu cljse and the other to open in the 

 intervals between the two groups cf light flashes. Approximately 25 pictures 1 inch in diameter may 

 be obtained on each drum. 



Each lens and shutter is mounted separately on a shaped brass base J which may be moved through 

 an arc of about 5° round the periphery of the drum. Thus one lens may be tilted upwards while the 

 other may be tilted downards. In Figure 5 the right hand lens is shown tilted downards, while the 

 brass dust cover has been removed to show the solenoid which operates the shutter. This/ arrangement of 

 tilting lenses enables two different fields of view, one above the other, to be covered, a feature 

 which may be used to advantage when the bubble is known to be moving either upwards or downwards. 



The film is held in position by simple spring clips L. The camera unit is made light-tight 

 by a 1 ight cover K. 



The camera proper is contained in a strong watertight housing which is made in two pieces. 

 The bed plate F of the camera bolts on to a heavy rigid plate M which in turn is bolted on to the 

 external framework holding the camera under water. The front portion of the watertight housing N 

 is welded on to the plate M. This portion has a compartment containing the motor C and a plate glass 

 window 0, 1 inch thick, is bolted between rubber ^gaskets to the front of N. This window has been 

 found to withstand the explosive effects of a 1 02, charge detonated at distances greater than 2 feet 

 6 inches from it. The rear portion of the housing consists of the watertight cover p which is also 

 belted on to the plate M to complete the seal. 



Lamp Housing . 



The Arditron lamp is mounted in a large car headlamp reflector in such a way that the beam 

 of light is concentrated roughly into a cone of about 50° semiangle. The 0.2-microfarad condenser is 

 mounted just behind the reflector and the whole is mounted in a wooden box held just above the water 

 surface. it has not been found necessary to use a glass plate in the water surface to prevent 

 scattering of light by ripples. 



Underuater Reflector . 



The construction of the curved metal reflector in Figure l, is shown in greater detail in 

 Figure 6. The basic framework consists of t(iree rows of upright angle irons A welded to thre^ cross- 



