73i 
Centrif7igal Force as Geo tropic Stimtili. 
value. And it usually happens that the point of equilibrium does not 
lie exactly in the middle of the box, but either above or below it. In 
Fig. 6 it lies 2 cm. below the middle of the box, and in taking a reading 
the centrifugal force at this point has to be worked out. 
Two Sources of Error. 
There are two main sources of error in these calculations : 
(i) The town-main current is not constant, and this gives rise to 
irregularity in the rate of rotation of the c wheel \ Such irregularities are 
shown in the last column on p. 732, where variations in rotation are shown 
from 3-59 revolutions per sec. to 375 revolutions per sec., and this within the 
course of twelve hours. I have found no method of correcting for this error. 
(ii) When the motor starts at the commencement of each period of 
rotation, it does not acquire its full speed for perhaps three or four seconds. 
Also, when the current ceases to flow, the c wheel ’ does not immediately 
stop. This causes an error in the estimated rate of rotation, worked out 
from the number of rotations completed and the actual time during which 
the current has been flowing. This error is most marked when the individual 
periods of rotation are short, so that the times taken by starting and stop- 
ping are longer compared with the time of even running ; and also when 
the bands round the pulleys are slack so that a considerable amount 
of slipping is allowed. When the individual periods of rotation are long, 
and the bands are tight, no correction need be made for this error. 
I have been able to compensate for this error, to some extent at 
any rate, in the following way. 
To take an example : the current flows for 30 seconds, but owing 
to the momentum acquired the ‘ wheel ’ rotates for 35 seconds, i. e. for 
5 seconds after the current has ceased to flow. In this total time (35 sec.) 
the ‘wheel’ makes 107 revolutions. Now, working out the value of the 
centrifugal force from the data of the control clock and the rotation 
recorder, we have 
d 7 r R 2 r 
R = W = 3-57 R2 = 12.74 and C = - g g — = 22-9 mg. 
since in this experiment the radius of rotation was 45 cm. A period was 
then carefully watched and the number of revolutions in each five seconds 
was recorded with the following result : 
1 st period of 5 sec. 
9 
revolutions 
R 2 - 32 
C= 6*7 
Ct — C x 5 = 
28*5 
2nd 
}j 
i 5 
3 > 
33 9 
33 16-2 
>> 
8i-o 
3rd 
j> 
17 
53 
,3 1 1-5 
33 20.7 
)) 
4 th 
33 
17 
33 
33 n-5 
33 20.7 
5 ) 
103*5 
5th 
33 
*7 
33 
,3 n-5 
,3 20-7 
) > 
103*5 
6th 
33 
i 7 
33 
33 ii-5 
3 , 20-7 
n 
103-5 
7 th 
3 ) 
1 2 
33 
3, 5-7 
33 10-2 
Y) 
51-0 
Total in Ct units 
574-5 
(The Ct unit represents a force of 1 mg. acting for one second). 
