Volltext: Elementary course in psychological measurements (4)

138 E. JV. Scripture, 
behind the others in order to keep the edges front flapping when they are 
b. Color 'wheel. The essential is a rapidly rotating axle with a screw- 
nut for fastening the discs against a flange. This is best obtained by using 
an electric motor. For the present exercise the motor is series-wound 
in order that its speed may be controlled by the amount of current. 
The current is brought to the two posts of the motor, passing in its way 
the two poles of a shunt switch. When the switch is closed the current 
passes directly across without going through the motor ; when it is 
open the current is forced to pass through the motor. 
c. Speed indicator. The rotation of the motor axle causes the indi¬ 
cator arms to revolve and to fly outward. The extent to which they 
move outward depends on the relation of the speed of revolution to the 
weight of the arms and the tension of the restraining springs. A pointer 
is connected with the indicator to show just how much the arms move. 
The pointer moves over a scale graduated to show the number of 
revolutions per second. The scale was established empirically by spark- 
records on the drum, after the manner of Exercise IX, the place of the key 
in that exercise being taken by a revolving pin at the end of the axle 
striking against a metal spring. 
d. Telescope. This may be a simple tube blackened inside ; it limits 
the amount of the disc seen to a circle of definite area. Or a simple 
reading telescope of the usual kind may be used. It is placed on the 
e. Adjusting the apparatus. The nut is removed from the color wheel. 
The backing disc is placed on the axle. The red and black discs are 
slipped together, so that when rotated they will not catch the wind ; if 
the motor rotates counter-clockwise the edges must overlap to the right. 
The discs thus slipped together are placed on the axle, a small paper ring 
is placed in front and the nut is partly screwed up. The circular gradu¬ 
ated scale is laid over the discs and they are moved till each occupies 
half the circle. The nut is then screwed tight. 
The battery is turned on and the motor started. 
The 32-c. p. lamp is lighted and placed at y2 meter in front and slightly 
below the discs. 
The Gauss tripod is placed at a distance of x meter from the motor. 
The telescope is then adjusted so that the eye sees the whole field of view 
as a colored circle. 
The motor being at rest and the subject looking through the telescope, 
the experimenter says “ Ready ” and sends the current through the motor.


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