Centrifugal Force as Geotropic Stimuli . 729 
rests on the support as shown in Text-fig. 3 ; but when one of the arms A lt 
A 2 , A 3 , A± touches the mercury M, as shown in the dotted line drawing, the 
electro-magnet E becomes alive and attracts the soft iron /, drawing the 
end d 1 down so as to touch the mercury in the mercury-bath m 1 . This 
connects the wire w lf which attaches the wire d, d 2 with the town main, and 
w 2 , which connects the mercury bath m 1 with the electric motor P, and 
starts the motor, since the other terminal of the motor is connected with 
the other terminal of the town main. The electric motor thus starts 
running and turns the ‘wheel’. So when one of the arms A ly A 2 , A 3 , A± 
touches the mercury bath M, the wheel starts rotating, and when the arm 
leaves the mercury the wheel stops. The object of the complication shown 
in portion III of this figure is that it makes and breaks the town main 
current very rapidly, and so prevents the lengthened sparking which 
occurred when the town main current was allowed to pass through the 
mercury bath M in portion II. 
An additional piece of apparatus was added in the form of a clock, 
which only worked whilst one of the arms A ly A 2y A 3 , A± was touching the 
mercury. This is shown in portion IV of the figure. A lever L is weighted 
at the end /, and the other end touches an extension attached to the balance- 
wheel of the clock and prevents it from ‘ going On this end of the lever 
is attached a soft iron plate, which lies above the electro-magnet E' . This 
electro-magnet is joined up in the same way as the electro-magnet E in 
portion III of the figure and is parallel with it. When the electro-magnet 
E' becomes active, the lever L is pulled down, freeing the balance-wheel of 
the clock and thereby starting the clock. Thus the clock only goes whilst 
the current is passing and the wheel rotating, so that the clock records the 
total time that the wheel has been rotating during the experiment. This 
clock will be referred to as the ‘ control clock \ 
The result of all this apparatus is that when any one of the arms A ly 
A 2 , A 3 , A± is in contact with the mercury M, the wheel is caused to rotate 
and the seedlings in the box are subjected to the centrifugal force. When 
the arm leaves the mercury the ‘ wheel ’ stops, and the seedlings are subject 
to gravity acting in the opposite direction. The alternation may be kept 
up as long as required (an experiment usually lasts from six to fifteen 
hours), and at the end of the time the control clock shows the total period 
during which the c wheel ’ was rotating, and the rotation recorder denotes 
the number of rotations performed by the ‘wheel’. From these data the 
average rate of rotation of the ‘ wheel ’, whilst moving, can be deduced 
and the centrifugal force worked out. The time during which gravity 
works alone is determined by subtracting the time recorded by the control 
clock from the total time of the experiment. 
