- 5 - 231 



is fixeO a knife-fflge K and e wire staple J passing over the briss b.-'.r attached tc H . When the upper 

 stuffing Dox is tis^ht and thi- lower one is slightly sl-ck.-ncd the framework can be moved vertically as a 

 whole or rotated. when the upper stuffing box is tight and the Kwer one is slightly slackened the 

 framework can be moved vertically as a whule „r rotated. When the upper stuffing b^x is slackened and 

 the lower jne is tight the central push rod can be mcved up cr d.-wn relative to the push tube, thus 

 raising ^.r Ijwering the knife-edge K and increasing or decreasing the gap between the ends of E and F. 



when photographing the bubble, it is essential t^. cnrrect the optical distrrtirn caused by the 

 cylindrical f.:rm ;f the bubble tank. F. r this purpose, twi parallelizing tranks, L, consisting of plate 

 glass plates cemented to metal frames which in turn ire cemented to the .;utside ^.f the tank, are placed 

 opposite L-ne another sc that the glass plates are vertical and parallel; water ^r .-i1 is poured Into 

 the parallelizing tanks so that the level >f the liquid is the same on thi; two sides of the glass cylinder. 



The inset in Figure 2 shows the n.;n-cadjustable spark gn( which was used in the early experiments 

 with water; this ga;: m s attache'! to a rod passing through a stuffing box in the lower steel plate, C, 

 and it is seen in the ;h.,tograph of Figure i. It was discarded because it offers toe large an obstruction 

 near the spark and because the ga,. was n.t adjustable. 



The general layout .f the apparatus is shown in Figure 3. In this cliagram, A represents the bubble 

 tank in ^'lan (with the end j-lates, etc., omitte.l), E anl F the electrodes forming the under-l iqui'J s,^ark gap 

 and L the glass-fronted tanks for correcting the distortion due to the cylindrical form of the bubble tank. 

 M represents the illuminating spark gap with electrodes set about 20/lOOOth inches apart; in experiments 

 with water, the electrodes were made of magnesium, which gives an intense spark with most of the energy 

 in the blue region of the spectrum, and blue-sensit ivo ("ordinary") photographic plates or films were used. 

 In experiments with our oil, which transmits only the- green , and yellow regions of tne spectrum, 7lnc 

 electrodes were used in conjunct ion with fast orthochromatic plates or films, 



The spark gap 1' is placed at the focus of 9 condenser lens, N, 6 inches diameter and 6 inches focal 

 length. The light from the illuminating spark thus passes through the bubble tank as a parallel beam; 

 after emerging from the vessel, it passes through a second lens P of high optical quality, which is set so 

 as to bring the light to a focus on the centre of the camera objective, g. The camera can be used either 

 as a stationary plate camera, ur, as shown in the diagram, fs a revolving drum camera. The camera lens Is 

 adjusted sc that the under-liquid spark gap is tharply focussed 1 n the phitographic plate or film. 



Figure 3 also shows the timing pendulum and the electrical circuits used to produce the explosion 

 and illuminating sparks. The pendulum, whose period is one second, consists of a steel rod fixed to a 

 horizontal rod mounted between accurately machined centres; at its lower end, the pendulum carries an 

 extension shaped as shown in the diagram. This extension swings above a number of brass screws with rounded 

 upper ends, the gap between the screws and the pendulum being adjusted to 4/iOOOth inches; these screws arc 

 mounted on an arc of insulating material and they are spaced so that the pendulum takes 5 milliseconds to 

 pass from any one screw to the next when the pendulum Is released from the horizontal. The first screw, 

 labelled In the diagram, determines the discharge of the condenser Cg used to produce the under-liquid 

 spark; the reneining screws, la6elled 1, 2, 3,.,. In the diagram, determine the discharge of the condenser 

 C- (of which only four are shown and only one labelled) used to produce the sequence of illuminating sparks 

 for photographing the bubble. 



AS the diagram shows, these condensers are connected in series with resistances across a large 

 reservoir condenser c^. charged through a valve rectifier to a potential difference of UOOO volts; the 

 junctions of the condensers and the series resistances are each connected to an electrode screw. 



When the pendulum swings over the screw 0, the gap between thf screw and the pendulum is momentarily 

 In series with the under-liquid spark gap; the two gaps break down and the condenser c discharges across 

 them. Similarly, the first condenser C] discharges across the illuminating gap and the gap between the 

 pendulum and the screw 1 when the pendulum moves over this screw, and so on. The 5 megohm resistances In 

 series with c in effect isolate this condenser and prevent more than one spark under the liquid during 

 the course of an experiment. The resistances Rj are such that the time-constants RjCj and R.C are large 

 enough to prevent the reservoir condenser c^ and the later Cj's from discharging completely during the early 

 illuminating sparks and, at the same time, small enough to allow a certain amount of charge to flow into 

 the condensers C- fr^m c,. 



