OK IMPACT WITH A LIQUID SURFACE. 
139 
circuit, one in the dark room for releasing the drop or sphere whose splash is to be 
photographed, and the other in the laboratory for releasing simultaneously the timing- 
sphere. 
A B is a light wooden rod, about 20 centims. in length, and rather stouter than a 
lead pencil; this is pivoted at C. At the end B is fixed a metal ring (B), on which tlie 
timing-sphere (T) can be placed. At the other end (A) is fixed a thin strip of tinned 
iron plate (D), which is held down by the electromagnet beneath ir against the 
pressure of a catapult made of an indiarubber ring stretched between the two hooks 
E and F. On cutting off the current of the electromagnet, the end A is tossed up by 
the catapult, and thus T is left in mid-air free to fall from rest. The end B is 
prevented from rebounding, and thus possibly interfering with the fall of T, by 
impaling itself on a suitably placed pin, which is hidden from view in this diagram, 
but is seen at H in fig. 2. 
The releasing key in the dark room is precisely similar to this, with the exception 
that when a liquid drop is to be let fall it carries, instead of the ring It, a small and 
deeply concave watch-glass, on which, when well smoked, a drop of milk or water will 
lie with little or no adhesion. 
On breaking the circuit of the two electromagnets the drop and timing-sphere are 
thus simultaneously released, and the former makes its splash at the moment that 
the latter, by passing between the fixed spherical terminals P and Q, insulated by 
supporting blocks of paraffin wax, discharges the Leyden-jars. 
The timing of the spark is effected by adjusting the height of fall of the timing- 
sphere, which is done by sliding the releasing key up or down its vertical supporting 
rod, the height being read off on a millimetre scale which was pinned against a firmly 
fixed and well planed, oak batten. 
The general arrangement of the apparatus is shown in fig. 2. 
It will be noticed that on the left-hand side of either figure there is an idle 
releasing lever. This projected over the edge of the table, and was used instead of 
the rightdiand lever when the height of fall was more than about 2 metres, the 
paraffin blocks carrying P and Q being then put on the floor. Instead of the rough 
electrometer figured, a pith ball, hanging by a cotton-thread from the upright stem 
of one of the jars, was more frequently employed to show the extent to which 
charging had proceeded, of which also the sound of brush discharges was a useful 
indication, for, in order to secure a bright spark, we generally charged the jars up to 
the limit determined by leakage. 
P]wtogra 2 )]iic A’pj^uratus and Details .—The chief necessity is to secure adequate 
illumination, and for this as little light as possible must be wasted. After many 
trials the difficulty was successfully met by using as a mirror a concave watch glass 
of a width about equal to that of the area to be illuminated, and very approximately 
parabolic in form and sufficiently deep for the focus to be very nearly in the plane of 
its circular edge. This glass was found to be suflhciently “ silvered ” by means of a 
T 2 
